build: drop the secp256k1-frost-kmp submodule

The previous commit moved the ChillDKG ritual onto bitcoin-kmp's native
implementation, which was the only thing in this app using this library. Nothing
imports `ac.cord.auxiliary.frost` or `ac.cord.auxiliary.cryptography` any more, so
the submodule, its composite build and the dependency on it all go.

Removed:
  - the `secp256k1-frost-kmp` submodule (deinit, git rm, and .git/modules cleared)
  - its entry in .gitmodules
  - `includeBuild("secp256k1-frost-kmp")` in settings.gradle.kts, with the
    surrounding comment made singular now that one composite build remains
  - `implementation("ac.cord.auxiliary:library:1.0.0")` in composeApp

Note that `ac/cord/auxiliary/compose/**` under composeApp/src/jvmMain is this
app's own code in a similarly-named package, unrelated to the library and
untouched.

Two commits made on the submodule to keep it building inside this composite --
gating its ios targets on macos, and moving it to AGP 9.4.0 -- existed only in
this repository's .git/modules copy and are destroyed by the removal. They were
exported as patches first. Neither is a loss worth chasing: both existed purely to
make that project cooperate with a composite build that no longer includes it.

The library is not gone from the world, only from this build. It still holds a
FROST signer, which is the obvious next need now that the ritual produces
threshold key material. But bitcoin-kmp ships `fr.acinq.bitcoin.crypto.frost.Frost`
against the same natives this app now packages, so the signer will almost
certainly come from there rather than from a second copy of secp256k1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "secp256k1-frost-kmp"]
-	path = secp256k1-frost-kmp
-	url = https://github.com/ac-cord-ac/secp256k1-frost-kmp.git
 [submodule "lightning-kmp-app"]
 	path = lightning-kmp-app
 	url = https://github.com/kngako/lightning-kmp-app.git

composeApp/build.gradle.kts

@@ -126,9 +126,6 @@ kotlin {
 
             implementation("com.ionspin.kotlin:bignum:0.3.10")
 
-            // resolved via the secp256k1-frost-kmp composite build (see settings.gradle.kts)
-            implementation("ac.cord.auxiliary:library:1.0.0")
-
             implementation("no.synth:kmp-zip:0.8.0")
             implementation("no.synth:kmp-zip-okio:0.8.0")
         }

settings.gradle.kts

@@ -34,8 +34,7 @@ plugins {
 
 include(":composeApp")
 
-// Composite builds: substitute the external module coordinates with the submodules' :library projects.
-includeBuild("secp256k1-frost-kmp")
+// Composite build: substitute the external module coordinate with the submodule's :library project.
 includeBuild("lightning-kmp-app") {
     dependencySubstitution {
         // the submodule's project stays named :library (compose-resources derives the Res package