Revert "Add pg_gin_pending_cleanup function."

This reverts commit 299f7f23c94f7674b780b1c3965201d3034bb6c9.

PostgreSQL 9.6 supports gin_clean_pending_list function which
provides the same functionality as pg_gin_pending_cleanup.
Since it's basically wasteful to maintain the same feature in
separate softwares, this commit drops pg_gin_pending_cleanup function.