Remove hg backfill code and references

[flodolo]

This week we had a 115 release based on the Git repository, so I think we're ready to stop backfilling the repos.

Once this is merged, my plan would be to remove the branches with this script

#!/bin/bash

branches=$(git branch -r | grep -v 'origin/main' | sed 's/origin\///')

for branch in $branches; do
  git push origin --delete "$branch"
done