CLN/REF: de-duplicate DatetimeTZBlock.setitem
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Move towards avoiding special treatment for pandas-internal EAs.

xref #24020
great, eliminating these special cases would be great (assume already on your list) > great, eliminating these special cases would be great (assume already on your list)

yah, basically all of my current core.internals PRs are aimed towards getting us down to just Block+ExtensionBlock