Check CirqBot and update as needed

[mhucka]

It is not clear whether CirqBot is doing things like labeling PRs with automerge. We need to check the setup and adjust as needed.

[pavoljuhas]

The automerge and front_of_queue_automerge were our own implementation of GitHub merge queue, which should be unnecessary as the stock queue seems to work well enough. Perhaps we should remove that functionality.

The bot still labels issues with size labels, (XS), (S), etc., which is useful.
We should keep that unless GitHub has added some equivalent action by now.

[mhucka]

Update: there does turn out to be some GitHub Actions in the GH Marketplace for labeling sizes of PRs. A workflow to emulate the behavior of pr_monitor for size labeling is in PR #7203.

After that PR is merged, we should be able to remove the pr_monitor code from Cirq, and also turn off the pr_monitor process on Google Cloud that CirqBot invokes to do the labeling.

It may also resolve #6024 and #3771.