DOC: Remove deprecated Y and M from to_timedelta
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

[`to_timedelta()`](https://github.com/pandas-dev/pandas/blob/master/pandas/core/tools/timedeltas.py#L84-L88) blocks those arguments anyway, so no reason to include them in the docstring.