Remove RTD PR preview build workflow (#2091)

We now use the [RTD GitHub app](https://docs.readthedocs.com/platform/stable/reference/git-integration.html#github-app), making the removed file redundant to pull request preview builds. Additionally, the call to the old GitHub Action was deprecated on 2025-07-01, per https://github.com/readthedocs/actions/blob/main/README.md.

Author: stevepiercy