buf_defer_drop_ahi(): Remove. Ever since
commit c7f8cfc9 (MDEV-27700)
it is safe to invoke btr_search_drop_page_hash_index(block, true)
to remove an orphan adaptive hash index.

Any attempt to upgrade page latches is prone to deadlocks. Recently,
we observed a few hangs that involved nothing more than a small table
consisting of one clustered index page, one secondary index page and
some undo pages.