Removed old xarr and ipython decorators
- [ ] progress towards #18190
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Didn't think either of these were used enough to justify as their own decorator, so I removed them from the old ``testing`` module and just used the generic ``skip_if_no`` decorator where applicable
thanks @WillAyd 