commit: remove unused inline function single_parent()

53b2c823f6 (revision walker: mini clean-up) added the function in 2007,
but it was never used, so we should be able to get rid of it now.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -313,11 +313,6 @@ extern int interactive_add(int argc, const char **argv, const char *prefix, int
 extern int run_add_interactive(const char *revision, const char *patch_mode,
 			       const struct pathspec *pathspec);
 
-static inline int single_parent(struct commit *commit)
-{
-	return commit->parents && !commit->parents->next;
-}
-
 struct commit_list *reduce_heads(struct commit_list *heads);
 
 struct commit_extra_header {