release steps: stop posting messages about branch cut in Slack

Created by: slimsag

Today our release tracking issue has us always do the following step 5 work days before the release:

• Post to #dev-announce the following message:

  :captain: *Release captain announcement:*
  
  Branch cut will be at the start of the next working day (2020-9-15).
  
  All changes that will be part of `3.20` (and all associated CHANGELOG updates) should be in `main` by tomorrow. Otherwise, they will not be included in the release.

This is redundant with what we post in GitHub issues already which spam's peoples email inboxes, and also makes releases eventful ("you have one day to merge your changes!!!") which I want to avoid. Let's stop doing this..