MDL-71747 upgrade: remove adhoc tasks not used anymore
This code was used only by deleted upgrade steps so it's safe
to proceed with straight deletion, considering it internal.
Deletion has been documented in corresponding upgrade.txt files:
- \mod_forum\task\refresh_forum_post_counts adhoc task
And these is the list of code that has NOT been removed
because it may be needed in the future:
- \core_search\manager::clean_up_non_existing_area(), used by
- adhoc task: \core\task\clean_up_deleted_search_area_task adhoc task