Clean up unused triggers in a single query for all dialects except MySQL

[hussein-awala]

This PR improves the performance of clean_unsed triggers method by executing the cleanup for Postgres and SQLite in a single query, which could reduce the latency and memory usage.

For MySQL, it doesn't support DELETE with JOIN, so we need to keep the current behavior:

[SQL: DELETE FROM `trigger` WHERE `trigger`.id IN (SELECT `trigger`.id 
FROM `trigger` LEFT OUTER JOIN task_instance ON `trigger`.id = task_instance.trigger_id GROUP BY `trigger`.id 
HAVING count(task_instance.trigger_id) = %s)]
[parameters: (0,)]
(Background on this error at: https://sqlalche.me/e/14/e3q8)