let RhythmicStaff use default barline extent (issue 35060046)

[k-ohara5a5a]

Looks good.

Repeat bars look fine (as they do in Drum Staff).

The book by Kurt Stone happens to have some bar-lines in single-line
percussion staves that are as tall as a 4/4 time-signature.  Other
examples in the same book have short bar-lines like these here, and
looking at some scores the short bar-lines are more common.

https://codereview.appspot.com/35060046/