Verify what happens with search areas on plugin removal

This comes from MDL-71747

We should check how we are handling, on plugin uninstall, all the information corresponding to search areas the plugin may have.

The point, raised from quen, is that we may need to use the \core\task\clean_up_deleted_search_area_task adhoc adhoc tasks that uses \core_search\manager::clean_up_non_existing_area() in those cases to guarantee that everything is properly removed.

Note that this issue is first, about to look what we are doing now (maybe we are already deleting everything properly) and, if not, ensure that we start doing it.

Ciao