amcheck: Remove unused IndexCheckableCallback typedef.

Commit d70b17636d introduced the IndexCheckableCallback typedef for
a callback function, but it was never used. This commit removes
the unused typedef to clean up dead code.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/e1ea4e14-3b21-4e01-a5f2-0686883265df@oss.nttdata.com

contrib/amcheck/verify_common.h

@@ -16,8 +16,7 @@
 #include "utils/relcache.h"
 #include "miscadmin.h"
 
-/* Typedefs for callback functions for amcheck_lock_relation_and_check */
-typedef void (*IndexCheckableCallback) (Relation index);
+/* Typedef for callback function for amcheck_lock_relation_and_check */
 typedef void (*IndexDoCheckCallback) (Relation rel,
 									  Relation heaprel,
 									  void *state,