[skip ci] Disable test running via comment (#15770)

There was an alpha feature in the repo, where all-post-commit runs could
be triggered by PR comment.

It wasn't working, because it was always triggering all-post-commit on
main, instead of the PR branch.

Also, users have reported accidentally triggering it.

Disabling until a later time when we want to add it.

Too many other things to fix right now.