Before this fix, one would get a 'Trigger ... already exists' when trying
to create a trigger matching the original name and 'Trigger ... does not
exists" when trying to drop it.
Fixes a reported bug in MDEV-25180 Atomic ALTER TABLE
MDEV-25517 Atomic DDL: Assertion `query_arg' in THD::binlog_query
upon DROP TRIGGER
The bug was that the stmt_query variable was not populated
with the query in case of DROP TRIGGER of an orphan trigger
(.TRN file exists & table exists, but the trigger was not in
table->triggers).
In main.index_merge_myisam we remove the test that was added in
commit a2d24def8c because
it duplicates the test case that was added in
commit 5af12e4635.
