Test-infra: Don't leave 9 "ok to test?" messages on every PR

[lavalamp]

I think each PR builder job leaves a comment saying this. We should not do this, it looks ridiculous...

[spxtr]

I agree, but we don't have a choice: https://github.com/jenkinsci/ghprb-plugin/issues/292

We can just not leave a message at all and have the munger try and figure it out, but there's not a good way to do it properly right now.

[eparis]

we could extend the munger's comment-deleter to remove 3 of them....

[spxtr]

Done :)