DOC: Remove doc spellcheck
The spellcheck system was implemented, but with dependencies (i.e. `enchant`) that are no longer maintained, and there is no reasonable way to set them up. This means that can't be added to the CI, and can't be even set up into our local environments with a decent amount of work installing dependencies manually (I didn't succeed).

The PR that was trying to finish the work on the spellcheck, #21402, was finally discarded, but part of the stuff is still in sphinx, just generating a warning when trying to get the sphinx extension.

This PR removes it.