simplify CBMonth.apply to remove roll_monthday
The implementations of CustomBusinessMonthBegin.apply and CustomBusinessMonthEnd.apply are merged.

Simplify CustomBusinessMonth.apply so that it uses liboffsets.roll_convention instead of liboffsets.roll_monthday (the latter being much more of a footgun).  This is the last place where roll_monthday was used, so this removes it.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
