Issue 185: Remove Pitch_squash_engraver (issue 35080044)

[src]

To make sure I understand correctly (unfortunately I cannot test the
patch right now, I only looked over the diffs): After this change,

- you can still freely mix squashed and unsquashed voices in one staff,
- to squash notes in one voice, you just set Voice.squashedPosition to
an appropriate numeric value, and
- to stop squashing again, you set it to #f (anything non-numeric
actually).

Correct?

If so, this is a nice simplification IMO.

https://codereview.appspot.com/35080044/