query: Make songs query less strict

The songs query was as strict as the albums and artists queries when it
came to tags being provided.

Make it less strict, so users can see and search all of their indexed
songs, even without the proper tags.

gnomemusic/query.py

@@ -208,9 +208,6 @@ class Query():
     {
         %(where_clause)s
         ?song a nmm:MusicPiece ;
-            nmm:musicAlbumDisc ?disc ;
-            nmm:musicAlbum ?album ;
-            nmm:performer ?performer ;
             nie:url ?url .
         OPTIONAL { ?song nao:hasTag ?tag .
                    FILTER (?tag = nao:predefined-tag-favorite) } .