TYP only set default for one overloaded function in reset_index
- [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

@simonjayhawkins I think we only need `=...` in one of the two overloads, then the error goes away and all passes:
```console
$ mypy pandas
Success: no issues found in 1244 source files
```
Sorry, this is wrong:
```python
# t.py

import pandas as pd

reveal_type(pd.DataFrame([1,2,3]).reset_index(inplace=True))
```

```console
$ mypy t.py 
t.py:3: error: No overload variant of "reset_index" of "DataFrame" matches argument type "bool"  [call-overload]
t.py:3: note: Possible overload variant:
t.py:3: note:     def reset_index(self, level: Union[Hashable, Sequence[Hashable], None] = ..., drop: bool = ..., inplace: Literal[False] = ..., col_level: Hashable = ..., col_fill: Hashable = ...) -> DataFrame
t.py:3: note:     <1 more non-matching overload not shown>
t.py:3: note: Revealed type is 'Any'
```

while on master
```console
$ mypy t.py 
t.py:3: note: Revealed type is 'None'
```