Add owner, file_id index to oc_photos_albums_files

[juliushaertl]

There are queries that get executed regularly on share removal which do not use an index:

https://github.com/nextcloud/photos/blob/c77b9046a17509ec166186f64e0dee934ddc3e9c/lib/Album/AlbumMapper.php#L320

Adding a combined index for owner, file_id should solve this