Problem:
--------
For DROP TABLE IF EXISTS and DROP DB IF EXISTS, Commit_order_manager::wait
and Commit_order_manager::wait_and_finish() were added in wl#7846, but they
are no more getting invoked now.
'DROP VIEW/FUNCTION/PROCEDURE/EVENT/TRIGGER IF EXISTS' were not tested
in wl#7846.

Solution:
---------
The Commit_order_manager::wait and Commit_order_manager::wait_and_finish()
added for DROP TABLE IF EXISTS and DROP DB IF EXISTS, no longer needed
and so removed.
Added tests for
'DROP VIEW/FUNCTION/PROCEDURE/EVENT/TRIGGER IF EXISTS'.

RB: 23274