iterator: remove unused function `tree_iterator_entry_cmp`

The function `tree_iterator_entry_cmp` has been introduced in commit be30387e
(iterators: refactored tree iterator, 2016-02-25), but in fact it has never been
used at all. Remove it to avoid unused function warnings as soon as we re-enable
"-Wunused-functions".

src/iterator.c

@@ -478,14 +478,6 @@ GIT_INLINE(int) tree_entry_cmp(
 		icase ? git__strncasecmp : git__strncmp);
 }
 
-GIT_INLINE(int) tree_iterator_entry_cmp(const void *ptr_a, const void *ptr_b)
-{
-	const tree_iterator_entry *a = (const tree_iterator_entry *)ptr_a;
-	const tree_iterator_entry *b = (const tree_iterator_entry *)ptr_b;
-
-	return tree_entry_cmp(a->tree_entry, b->tree_entry, false);
-}
-
 GIT_INLINE(int) tree_iterator_entry_cmp_icase(
 	const void *ptr_a, const void *ptr_b)
 {