CLN: Comparison methods for MultiIndex should have consistent behaviour for all nlevels  (GH21149)
- [x] closes #21149 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Initial PR #21182 is closed - and now split in 2 different PRs

`def cmp_method` raised ValueError for equality / inequality comparisons of MultiIndex with nlevels == 1, which was inconsistent with behaviour for MultiIndex with nlevels > 1 (details of issue below) - this has now been fixed

Currently (as of 0.23.0), comparing MultiIndex of nlevels==1 with another of same length raises a ValueError e.g.
```python
[In] midx=pd.MultiIndex.from_product([[0, 1]])
[In] midx
[Out] MultiIndex(levels=[[0, 1]],
           labels=[[0, 1]])
[In] midx == midx
[Out] ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
whereas the behaviour should be consistent with that for MultiIndex with nlevels>1 as follows:
```python
[In] midx == midx
[Out] array([ True,  True])
```