Fix bug with stuck notes when challenge would switch staves.

Fix bug with stuck notes when challenge would switch staves.

In this case we were failing to remove a note beacuse we were looking
at the new staff (where the new challenge is) rather than the old staff
where the note was originally created.

Fix this by having each note remember the staff it's created on. This
also impacts the API a bit (no longer need a staff argument to
remove_note, so drop the "_staff" portion of the name from both
add_note and remove_note).