Issue 5934: remove Repeated_music::folded_music_length (issue 561670043 by nine.fierce.ballads@gmail.com)

[dak]

Interesting.  In 2.18 it appears to be used as length-callback in
TremoloRepeatedMusic.  Was that wrong or did something change in
unfolded_music_length ?

https://codereview.appspot.com/561670043/