commit: remove find_header_mem()

cfc5cf428b (receive-pack.c: consolidate find header logic, 2022-01-06)
introduced find_header_mem() and turned find_commit_header() into a thin
wrapper.  Since then, the latter has become the last remaining caller of
the former.  Remove it to restore find_commit_header() to the state
before cfc5cf428b, get rid of a strlen(3) call and resolve a NEEDSWORK
note in the process.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -280,17 +280,12 @@ void free_commit_extra_headers(struct commit_extra_header *extra);
 
 /*
  * Search the commit object contents given by "msg" for the header "key".
- * Reads up to "len" bytes of "msg".
  * Returns a pointer to the start of the header contents, or NULL. The length
  * of the header, up to the first newline, is returned via out_len.
  *
  * Note that some headers (like mergetag) may be multi-line. It is the caller's
  * responsibility to parse further in this case!
  */
-const char *find_header_mem(const char *msg, size_t len,
-			const char *key,
-			size_t *out_len);
-
 const char *find_commit_header(const char *msg, const char *key,
 			       size_t *out_len);