Update frame.py
If not all levels are removed, the index is not reset to default integer values.

import pandas as pd

x = pd.Series(range(3), index=pd.MultiIndex.from_arrays([['a', 'b', 'c'], [1, 2, 3]], names=['letters', 'numbers']))

print(x.reset_index('numbers', drop=True))

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Hello @cf-vrgl! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/frame.py`](https://github.com/pandas-dev/pandas/blob/30fc3eeff5bab70b37658759988b957f7a39f366/pandas/core/frame.py):

> [Line 5618:74](https://github.com/pandas-dev/pandas/blob/30fc3eeff5bab70b37658759988b957f7a39f366/pandas/core/frame.py#L5618): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace

