Merge bitcoin/bitcoin#25281: build: Remove unused LIBBITCOIN_KERNEL variable

ce1c810 build: Remove unused `LIBBITCOIN_KERNEL` variable (Hennadii Stepanov)

Pull request description:

  Noticed that while working on moving the build system to CMake. But I [am not the first](https://github.com/bitcoin/bitcoin/pull/24322/files#r860472867) one :)

ACKs for top commit:
  laanwj:
    ACK ce1c810

Tree-SHA512: 877b9f0d64c4c72f403335d7a8462e551f6f8cd5648a211f980d6da5ed7683521d6549f6acf15ac8e55f67915c556201a1980228c975a22135507746e2f392ce

Author: laanwj

src/Makefile.am

@@ -29,7 +29,6 @@ LIBBITCOIN_NODE=libbitcoin_node.a
 LIBBITCOIN_COMMON=libbitcoin_common.a
 LIBBITCOIN_CONSENSUS=libbitcoin_consensus.a
 LIBBITCOIN_CLI=libbitcoin_cli.a
-LIBBITCOIN_KERNEL=libbitcoin_kernel.a
 LIBBITCOIN_UTIL=libbitcoin_util.a
 LIBBITCOIN_CRYPTO_BASE=crypto/libbitcoin_crypto_base.la
 LIBBITCOINQT=qt/libbitcoinqt.a