Replace `wire::Message`'s `Writeable` implementation with `unreachable!` #2450

Job	Run time
benchmark	1m 17s
build-tx-sync (ubuntu-latest, beta)	1m 18s
build-tx-sync (macos-latest, stable)	1m 19s
check_release	1m 17s
build-tx-sync (ubuntu-latest, 1.75.0)	1m 18s
build-tx-sync (macos-latest, beta)	1m 20s
build-tx-sync (ubuntu-latest, stable)	1m 17s
build (macos-latest, 1.75.0)	1m 20s
build (windows-latest, stable)	1m 17s
build (self-hosted, beta)	1s
build (self-hosted, stable)	1s
build-tx-sync (macos-latest, 1.75.0)	1m 20s
build (self-hosted, 1.75.0)	1s
check_docs	1s
fuzz	1s
build (macos-latest, stable)	1m 22s
ext-test	1s
linting	1m 17s
rustfmt	17s
coverage	1s
	16m 6s

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