ui-shared: remove "format" from cgit_print_age()

We never use any format other than FMT_SHORTDATE, so move that into the
function.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-repolist.c

@@ -79,7 +79,7 @@ static void print_modtime(struct cgit_repo *repo)
 {
 	time_t t;
 	if (get_repo_modtime(repo, &t))
-		cgit_print_age(t, -1, NULL);
+		cgit_print_age(t, -1);
 }
 
 static int is_match(struct cgit_repo *repo)