Metadata: remove obsolete comparison against Unknown strings

Once upon a time, TrackInfo objects used to gather Unknown-like strings
("Unknown Album", "Unknown Artist", ...) when the tracks had these fields
blank, so then other Banshee services had to adapt to this strategy in
order to avoid dealing with these blank fields, so then this string
comparisons were hardcoded in them.

However, since 2009, this situation changed [1], so these Unknown strings
were only present in the Display* properties (DisplayAlbumTitle,
DisplayArtistName), therefore there is no need anymore for MetadataService
and CoverArtJob classes to do these string comparisons, and they will
just call String.IsNullOrEmpty().

[1] https://git.gnome.org/browse/banshee/commit/?id=7b201dd668ff833f8ebc61a75d4d1a020e5a724a