commit: remove unused forward declaration

In 6206089 (commit: write commits for both hashes, 2023-10-01),
`sign_with_header()` was removed, but its forward declaration in
"commit.h" was left. Remove the unused declaration.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jltobler

commit.h

@@ -400,8 +400,6 @@ LAST_ARG_MUST_BE_NULL
 int run_commit_hook(int editor_is_used, const char *index_file,
 		    int *invoked_hook, const char *name, ...);
 
-/* Sign a commit or tag buffer, storing the result in a header. */
-int sign_with_header(struct strbuf *buf, const char *keyid);
 /* Parse the signature out of a header. */
 int parse_buffer_signed_by_header(const char *buffer,
 				  unsigned long size,