ref-filter: remove ref_format_clear()

Now that ref_format_clear() no longer releases any memory we don't need
it anymore.  Remove it and its counterpart, ref_format_init().

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

builtin/verify-tag.c

@@ -69,6 +69,5 @@ int cmd_verify_tag(int argc,
 		if (format.format)
 			pretty_print_ref(name, &oid, &format);
 	}
-	ref_format_clear(&format);
 	return had_error;
 }