Fix code incompatible with sql_mode=only_full_group_by

[PhantomWatson]

CommunitiesTable::findAdminIndex() has a GROUP BY clause that appears incompatible with sql_mode=only_full_group_by. This method isn't actually used, so it's not currently generating any error to confirm that.

Options:

• Remove this unused method so it can't hurt anyone
• Replace ->group('Communities.id') with different code that's compatible with only_full_group_by
• Reconfigure MySQL to disable only_full_group_by