Fix music load lag by reverting tag changes

Since public-musicplus-core was merged, music would load while freezing the game for a moment. Most noticeable with jingles.

This happens because I changed the tag reading. So this merge has two commits:

1. Removed MP3 tag reading. It's slow, and it's wrong according to ID3 spec.
2. Removed LENGTHMS= tag. This is only an issue because all current songs don't have this tag, which causes the code to search the entire lump from beginning to end.

This reverts the tag behavior to what it was before, and resolves the skip-lag.

LENGTHMS= isn't really necessary for now, but if jingle resuming is implemented, the game will need a way to grab the song's length. Maybe it can be a field in MUSICDEF