Delete the `libstdcxx-ng` Patch in the Documentation Build

The documentation build is currently still doing the `libstdcxx-ng` patch, which is no longer necessary. Just need to remove [this step](https://github.com/ReactionMechanismGenerator/RMG-Py/blob/1d67d9405fc044952ac713a00dffe88d06982db2/.github/workflows/docs.yml#L19).