remove unused readthedocs workflow (#1046)

#721 added a GitHub Actions workflow that runs whenever changes are pushed to `branch-0.20`, as a workaround for some readthedocs failures.

There hasn't been a push to `branch-0.20` in 3 years (https://github.com/rapidsai/ucx-py/tree/branch-0.20) and readthedocs builds have been working well:

* builds: https://readthedocs.org/projects/ucx-py/builds/
* docs: https://ucx-py.readthedocs.io/en/latest/

So I believe this workflow is no longer necessary.

This proposes removing it.

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Peter Andreas Entschev (https://github.com/pentschev)
  - Bradley Dice (https://github.com/bdice)

URL: #1046