CLN/API: clean-up of level argument of isin for indexes
The docstring for Index.isin() states:

```
        level : str or int, optional
            Name or position of the index level to use (if the index is a
            `MultiIndex`).
```

But the keyword is accepted by the other indexes, validated and raises if the level parameter does not match the only level in the index.

This PR removes the levels argument from the indexes (except MultiIndex)

Series.isin() and DataFrame.isin() do not have a level parameter, so isin for indexes will be consistent.

The implementation for MultiIndex.isin is independent from Index.isin, so i think that a separate docstring for MultiIndex would be much clearer. (but out of scope for this PR)

The docstring for datetimelike indexes does not include the level parameter but is not part of the published api, so has been removed to avoid confusion.

NOTE: This is not a change to the api, but a clean-up to match the api to the published api.