AudioTrack: Remove UNIQUE(media_id, attached_file_id) constraint

The code explicitly tries to insert multiple tracks from a single
external file. In the unlikely event this would occur, it would trigger
a constraint violation.

refs #392
12 jobs for !543 with remove_audio_track_file_unique_constraint in 12 minutes and 54 seconds (queued for 1 minute and 45 seconds)
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