write_ref_sha1(): remove check for lock == NULL

None of the callers pass NULL to this function, and there doesn't seem
to be any usefulness to allowing them to do so.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -3080,10 +3080,6 @@ static int write_ref_sha1(struct ref_lock *lock,
 	static char term = '\n';
 	struct object *o;
 
-	if (!lock) {
-		errno = EINVAL;
-		return -1;
-	}
 	if (!lock->force_write && !hashcmp(lock->old_sha1, sha1)) {
 		unlock_ref(lock);
 		return 0;