trim audio for ScreenHowToPlay

This file only needs to contain a single loop of music; the StepMania engine will take care of the rest.  This cuts an additional 2 MB out of the project's overall filesize.

It's worth noting that you don't *have* a ScreenHowToPlay defined in your metrics.ini right now.  Assuming you intend to implement one, you should accept this pull request.

If you don't intend to implement that screen, you can close this pull request, but you should still delete the original *./Sounds/ScreenHowToPlay music (loop).mp3*