idxmap: remove unused foreach macros

The foreach macros of the idxmap types are not used anywhere. As we are
about to open-code all foreach macros for the maps in order to be able
to make the khash structure internal, removing these unused macros will
leave a few places less that need conversion.

src/idxmap.h

@@ -49,7 +49,4 @@ void git_idxmap_icase_delete_at(git_idxmap_icase *map, size_t idx);
 void git_idxmap_delete(git_idxmap *map, const git_index_entry *key);
 void git_idxmap_icase_delete(git_idxmap_icase *map, const git_index_entry *key);
 
-#define git_idxmap_begin		kh_begin
-#define git_idxmap_end		kh_end
-
 #endif