[Chapel Merge] Remove the Chapel sphinx domain snapshot

Branch: refs/heads/master
Revision: d2509d9
Author: lydia-duncan
Log Message:

Merge pull request #16683 from lydia-duncan/removeSphinxDomain

Remove the Chapel sphinx domain snapshot
[reviewed by @mppf]

This was committed because we weren’t as principled about pinning to the Sphinx
domain version, so an earlier update to the Chapel sphinx domain repo would have
broken chpldoc in previous releases. Now that we’ve been able to update that
repo, we can pin to the current version and update when we need to, so we don’t
need a check out (and don’t have to worry about keeping the two places in sync)

Tested by performing a make clobber in third-party/chpl-venv, rebuilding chpldoc,
and running start_test over test/chpldoc on my Mac. Full paratest on linux passed.