Notification Schema Fixes

Notifications tables are dropped because phpBB currently does not have
any way to make the necessary changes to the DB schema (and no release
has yet been made with these changes).

This will fix the following bugs:
http://tracker.phpbb.com/browse/PHPBB3-11411
http://tracker.phpbb.com/browse/PHPBB3-11413
http://tracker.phpbb.com/browse/PHPBB3-11416