Purging "PMEDIA" spam from my music library with Beets and Mutagen

I have been cleaning up my music collection, which I have ripped from CDs over the years, finally getting around to tagging them properly. I used MusicBrainz Picard to tag my music, but what I didn&rsquo;t know is that it was adding a bunch of tags which contain &lsquo;PMEDIA&rsquo; (even in places like the composer field, which makes no sense). Seeing this as annoying, I needed a way to clean up…