CLN: Remove Categorical.itemsize
The method doesn't even work in main, it is useless and removal keeps the same error code as before, so I think it's OK to just remove and not do a deprecation cycle here:

```python
>>> cat = pd.Categorical([9, 6])
>>> cat.itemsize
AttributeError: 'Int64Index' object has no attribute 'itemsize'  # main
AttributeError: 'Categorical' object has no attribute 'itemsize'  # new
```


Looks good after the merge conflict is resolved Rebased. Thanks @topper-123 