[ticket/13238] Do not drop indexes that do not exist from fulltext search

https://tracker.phpbb.com/browse/PHPBB3-13238

The reason why this one works, in opposite to #3092 is, because we only get the list of indexes in the `effectively_installed()`.
This method is not called when generating the schema. So the schema can still be generated with the wrong db object. For live boards this does not matter, because when the update is run, `effectively_installed()` is called, filling the array with the indexes that need to be deleted:
https://github.com/phpbb/phpbb/blob/develop/phpBB/phpbb/db/migrator.php#L254
https://github.com/phpbb/phpbb/blob/develop/phpBB/phpbb/db/migrator.php#L283