Fixed: Removed extraneous TODO

While investigating this TODO I realized that it's a problem in the MEI file,
not in my importer. I can't fix it either, so I just removed the TODO.
MEI: Remove _postGuessTuplets() in preparation for new tuplet-guessing

This new tuplet-guessing algorithm will happen at the <layer> level. It's
much faster, less complicated, more reliable, and coming up soon!
MEI: New tuplet-guessing algorithm at the <layer> level

Compared to the previous algorithm, this is fast, correct, and slick.

So far, it only works for tuplets that start and end in the same measure.