Extend undef op to work on fully qualified symbols

[yuhan0]

Before submitting a PR make sure the following things have been done:

• The commits are consistent with our contribution guidelines
• You've added tests to cover your change(s)
• All tests are passing
• The new code is not generating reflection warnings
• You've updated the README (if adding/changing middleware)

See clojure-emacs/cider#2814.
The current cider.el frontend sends undef op with ns as the current ns and symbol as a possibly ns-qualified var, which this PR interprets as unmapping the symbol from the corresponding other namespace.

For some reason running tests locally with make test turns up many failures in cider.nrepl.middleware.debug-integration-test which seem to be unrelated to the changes in this PR.

[bbatsov]

The current cider.el frontend sends undef op with ns as the current ns and symbol as a possibly ns-qualified var, which this PR interprets as unmapping the symbol from the corresponding other namespace.

This looks reasonable to me.
I'd suggest documenting the update behaviour in the ns docstring (high-level) and the undef function's docstring.

This should also be mentioned in the changelog.

[bbatsov]

Thanks for tackling this! 🙇