Removed old xarr and ipython decorators

[WillAyd]

• progress towards TST: make _skip_if into pytest decorators #18190
• tests added / passed
• 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

[gfyoung]

Sounds good to me!

[jreback]

thanks @WillAyd