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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@9d8dbef`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19146/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19146   +/-   ##
=========================================
  Coverage          ?   91.51%           
=========================================
  Files             ?      148           
  Lines             ?    48782           
  Branches          ?        0           
=========================================
  Hits              ?    44641           
  Misses            ?     4141           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.88% <100%> (?)` | |
| #single | `41.61% <19.04%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/19146/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.98% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=footer). Last update [9d8dbef...4ee7c82](https://codecov.io/gh/pandas-dev/pandas/pull/19146?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
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