bip-0054: rephrase/dedup motivation and rationale for BIP 34 cleanup

The rationale was duplicating some of the motivation. The motivation had a sentence that read weird.
While rephrasing the sentence, take the opportunity to link to the now-proposed Utreexo BIP. Also
remove a duplicate link reference.

bip-0054.md

@@ -45,12 +45,10 @@ any other user of Merkle proofs, to rely on one of the available workarounds[^13
 necessary in the first place.
 
 Since [bip-0034][BIP34] activation, explicit [bip-0030][BIP30] validation is not necessary until
-block height 1,983,702[^0].  Mandating new coinbase transactions be different from the early
+block height 1,983,702[^0]. Resuming [bip-0030][BIP30] validation would unnecessarily increase block
-[bip-0034][BIP34] violations makes it possible to get rid of [bip-0030][BIP30] validation forever.
+validation overhead and preclude alternative full node designs (such as [bip-0182][BIP182] Utreexo).
-Besides its unnecessary cost, another downside of [bip-0030][BIP30] validation is that it cannot be
+Enforcing that new coinbase transactions are different from the early [bip-0034][BIP34] violations
-performed by Utreexo clients. Finally, leveraging the coinbase transaction's `nLockTime` field
+makes it possible to get rid of [bip-0030][BIP30] validation forever.
-allows applications to recover the block height corresponding to a coinbase transaction without
-having to parse Script.
 
 ## Specification
 
@@ -114,17 +112,13 @@ invalidating 64-byte transactions, fixing the vulnerability without Merkle proof
 rely on any workaround or even know one is necessary in the first place. See [this post][64 bytes
 debate] for an attempt at summarizing the arguments for both sides of this debate.
 
-Several blocks prior to [bip-0034][BIP34] activation contain a coinbase transaction whose scriptSig
+The `nLockTime` field of transactions is a natural place to store a block height and is currently
-contains a valid [bip-0034][BIP34] commitment to a future block height. This offers an opportunity
+unused in coinbase transactions. Using it to enforce that new coinbase transactions differ from
-to duplicate these coinbase transactions in the future[^10] and for this reason [bip-0030][BIP30]
+early [bip-0034][BIP34] violations also allows applications to recover the block height without
-validation will need to be re-activated from block 1,983,702. A simple way to prevent this is to
+having to parse Script. Leveraging the existing timelock mechanism makes the check self-contained:
-mandate that future coinbase transactions vary from coinbase transactions before [bip-0034][BIP34]
+the same coinbase transaction cannot have been valid in a previous block[^11]. This simplifies both
-activation. There are multiple ways of achieving this, but setting and enforcing the timelock for
+reasoning and client implementation, since the [bip-0030][BIP30] check can be skipped entirely past
-the coinbase transaction makes it so all coinbase transactions past Consensus Cleanup activation
+Consensus Cleanup activation, regardless of the [bip-0034][BIP34] activation status[^12].
-could not have been valid before this height and therefore cannot be a duplicate[^11]. This
-simplifies both reasoning and client implementation, since the [bip-0030][BIP30] check can be
-skipped entirely past Consensus Cleanup activation, regardless of the [bip-0034][BIP34] activation
-status[^12].
 
 ## Backward compatibility
 
@@ -215,8 +209,6 @@ implemented caching that made it vulnerable to this attack. See [this writeup][S
 Suhas Daftuar for a detailed explanation. Invalidating 64-byte transactions may avoid this risk, but
 the issue is largely orthogonal to this proposal: it is fundamentally about caching validation
 status for malleable blocks.
-[^10]: See [here][BIP34 list] for a full list of the heights of historical blocks including a valid
-bip-0034 height commitment and the corresponding future block height.
 [^11]: Technically it could be argued a duplicate could in principle always be possible before block
 31,001 when `nLockTime` enforcement [was originally soft-forked][Harding nLockTime]. But treating
 coinbase transactions as not having duplicate past Consensus Cleanup activation would be consistent
@@ -236,6 +228,7 @@ MERKLEBLOCK] for a full description.
 
 [Delving worst block]: https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/93
 [BIP30]: https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki
+[BIP182]: https://github.com/bitcoin/bips/pull/1923
 [BIP-XXXX]: https://github.com/TheBlueMatt/bips/blob/7f9670b643b7c943a0cc6d2197d3eabe661050c2/bip-XXXX.mediawiki
 [BIP34]: https://github.com/bitcoin/bips/blob/master/bip-0034.mediawiki
 [BIP16 specs]: https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki#specification
@@ -251,7 +244,6 @@ MERKLEBLOCK] for a full description.
 [Sergio post]: https://bitslog.com/2018/06/09/leaf-node-weakness-in-bitcoin-merkle-tree-design
 [Sergio MERKLEBLOCK]: https://bitslog.com/2018/08/21/simple-change-to-the-bitcoin-merkleblock-command-to-protect-from-leaf-node-weakness-in-transaction-merkle-tree/
 [64 bytes debate]: https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/41
-[BIP34 list]: https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/4
 [Harding nLockTime]: https://bitcoin.stackexchange.com/questions/90229/nlocktime-in-bitcoin-core
 [Delving duplicable]: https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/4
 [Core 0.16.1]: https://bitcoincore.org/en/releases/0.16.1