This commit removes the Slack output every time security issues and merge
requests are processed.

Why?
* We have a better (and more reliable) way to visualize security issues
  that were processed and that are pending. Example:
https://gitlab.com/gitlab-org/gitlab/-/issues/393774#note_1326214053.
* The Slack output is unreliable, it doesn't display what issues are
  deployed to master nor if the backports are processed.
* The Slack output is also ephemeral, after 90 days the messages will be
  deleted.
* Printing Slack output involves a lot of code, this is code the
  Delivery team has to continuously maintain.

In general, we should focus our efforts on improving the table that is
posted in the tracking issue, deleting the code associated with the old
way is the first step for that.