Remove the UxPaperCutsMrs processor

Context

I've seen this error in triage-ops logs:

The slack API returned an error: channel_is_archived (HTTP Code 410)
Check the "Handling Errors" section on https://api.slack.com/incoming-webhooks for more information
/srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger/util/http_client.rb:29:in `block in call'
  <internal:kernel>:90:in `tap'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger/util/http_client.rb:27:in `call'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger/util/http_client.rb:13:in `post'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger.rb:49:in `block in post'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger.rb:47:in `map'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger.rb:47:in `post'
  /srv/rack/triage/vendor/bundle/ruby/3.2.0/gems/slack-messenger-2.3.6/lib/slack-messenger.rb:37:in `ping'
  /srv/rack/triage/processor/ux_paper_cuts_mrs.rb:50:in `post_merged_mr_to_slack'
  /srv/rack/triage/processor/ux_paper_cuts_mrs.rb:29:in `process'
  /srv/rack/triage/triage/processor.rb:73:in `block (2 levels) in triage'

It looks like the #ux_paper_cuts_mrs slack channel was archived last week.

Given Since the processor's sole purpose is to create Slack notifications, it feels like we can remove it.

What does this MR do and why?

Delete the UxPaperCutsMrs processor.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review