[bdk_chain_redesign] Rm unnecessary code and premature optimisation

* Remove `chain_oracle::CacheBackend` for now as it is not used.
* `SparseChain` does not need to implement `ChainOracle`.
* Remove filter predicate for `list..` methods of `TxGraph` and
  `IndexedTxGraph` as this is premature optimisation.
* `Append` can be implemented for all `BTreeMap`s and `BTreeSet`s,
  instead of only `local_chain::ChangeSet`.

crates/chain/src/chain_oracle.rs

@@ -1,9 +1,3 @@
-use crate::collections::HashSet;
-use core::marker::PhantomData;
-
-use alloc::{collections::VecDeque, vec::Vec};
-use bitcoin::BlockHash;
-
 use crate::BlockId;
 
 /// Represents a service that tracks the blockchain.
@@ -25,53 +19,3 @@ pub trait ChainOracle {
         static_block: BlockId,
     ) -> Result<Option<bool>, Self::Error>;
 }
-
-/// A cache structure increases the performance of getting chain data.
-///
-/// A simple FIFO cache replacement policy is used. Something more efficient and advanced can be
-/// implemented later.
-#[derive(Debug, Default)]
-pub struct CacheBackend<C> {
-    cache: HashSet<(BlockHash, BlockHash)>,
-    fifo: VecDeque<(BlockHash, BlockHash)>,
-    marker: PhantomData<C>,
-}
-
-impl<C> CacheBackend<C> {
-    /// Get the number of elements in the cache.
-    pub fn cache_size(&self) -> usize {
-        self.cache.len()
-    }
-
-    /// Prunes the cache to reach the `max_size` target.
-    ///
-    /// Returns pruned elements.
-    pub fn prune(&mut self, max_size: usize) -> Vec<(BlockHash, BlockHash)> {
-        let prune_count = self.cache.len().saturating_sub(max_size);
-        (0..prune_count)
-            .filter_map(|_| self.fifo.pop_front())
-            .filter(|k| self.cache.remove(k))
-            .collect()
-    }
-
-    pub fn contains(&self, static_block: BlockId, block: BlockId) -> bool {
-        if static_block.height < block.height
-            || static_block.height == block.height && static_block.hash != block.hash
-        {
-            return false;
-        }
-
-        self.cache.contains(&(static_block.hash, block.hash))
-    }
-
-    pub fn insert(&mut self, static_block: BlockId, block: BlockId) -> bool {
-        let cache_key = (static_block.hash, block.hash);
-
-        if self.cache.insert(cache_key) {
-            self.fifo.push_back(cache_key);
-            true
-        } else {
-            false
-        }
-    }
-}