Don't ping MR coaches when 'workflow::ready for review' is added

What does this MR do and why?

This changes the AutomatedReviewRequestGeneric processor to stop pinging a random MR coach automatically when workflowready for review is added, as proposed in #1023 (closed).

This is a small improvement that will avoid annoyingly pinging coaches as it's not always desirable, e.g.

Instead, we'll regularly ping assigned reviewers to take action on the MRs they're assigned to (!1387 (closed)).

Closes #1023 (closed).