Remove non-existent columns from iTunes library

The table 'itunes_library' does not contain 'album_artist' and
'grouping' columns yet! Reverts changes to this file that have
been applied by merging the branch master_AlbumArtistGroupingTag.
Merge pull request #94 from uklotzde/master_AlbumArtistGroupingTag_iTunesLibraryHotfix

Remove non-existent columns from iTunes library