Catch IntervalIndex.itemsize deprecation warning in tests and remove IntervalArray.itemsize
- [X] closes #22049
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I don't think a whatsnew entry is needed.  This was already deprecated for `IntervalIndex`, but wasn't being caught in the tests, and was displaying the name from the underlying `left`/`right` values.  Likewise, `IntervalArray` was created after the deprecation, so no reason to even add it.
