Remove Scatter and Hexbin from Series plot documentation
- [x] closes #38976
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] Remove scatter and hexbin entry from https://github.com/pandas-dev/pandas/blob/v1.2.0/pandas/plotting/_core.py#L603-L1708
