bpf, lsm: Remove bpf_lsm_key_free hook

The key_free LSM hook has been removed.
Remove the corresponding BPF hook.

Avoid warnings during the build:
  BTFIDS  vmlinux
WARN: resolve_btfids: unresolved symbol bpf_lsm_key_free

Fixes: 5f8d28f ("lsm: infrastructure management of the key security blob")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20241005-lsm-key_free-v1-1-42ea801dbd63@weissschuh.net

Author: t-8ch

kernel/bpf/bpf_lsm.c

@@ -339,10 +339,6 @@ BTF_ID(func, bpf_lsm_path_chmod)
 BTF_ID(func, bpf_lsm_path_chown)
 #endif /* CONFIG_SECURITY_PATH */
 
-#ifdef CONFIG_KEYS
-BTF_ID(func, bpf_lsm_key_free)
-#endif /* CONFIG_KEYS */
-
 BTF_ID(func, bpf_lsm_mmap_file)
 BTF_ID(func, bpf_lsm_netlink_send)
 BTF_ID(func, bpf_lsm_path_notify)