Track featured artists don't indicate compilation

This fix only uses primary track artist to determine whether an album is
a compilation. Featured artists no longer influence this.
Remove unnecessary comment
Compilation if set in MB releasegroup releasetype
Merge pull request #159 from Sophist-UK/sophist_fix_compilation_identification

Fix compilation flag incorrectly set by Picard, track featured artists don't indicate compilation.

Sophist explained:
Ideally the MB database would show a secondary type of Compilation accurately, but since it (presumably) doesn't, Picard tries to determine whether it is a compilation by seeing if all the tracks are by the same artist, the implication being that it is a compilation album if there are tracks with different artists.

There are a few problems with this:

1. Picard was comparing both primary and secondary track artists, so if all tracks have the same main artist but some also have featured artists (which is NOT a compilation), then Picard was flagging this (incorrectly) as a compilation.

2. Picard was not checking the release-group secondary-type field for Compilation - so e.g. a Greatest Hits album is a compilation, but since often all tracks have the same artist, Picard was flagging this (incorrectly) as NOT a compilation.