Drop support for treating MusicBrainz ids as track unique ids

No comments on my mail inquiry, which I treat as a consent. Since 2.6
the SqlScanResultProcessor should cope with changed track ids just fine.

CHANGES:
 * Problematic support for treating MusicBrainz ids as track unique ids
   was dropped; should avoid surprising "Duplicate Tracks Found" errors.

@Alberto, you'll probably need to rebase your patch as I've touched
MusicBrainzFinder.cpp

BUG: 315329
GUI: Mentions of MusicBrainz ids being useful for AFT (Amarok File Tracking)
     in various documentation need to be removed
CCMAIL: amarok-devel@kde.org
CCMAIL: Alberto Villa <avilla@FreeBSD.org>