Remove Gitter because it's no longer under GitLab

Remove Gitter because it's no longer under GitLab. It is now moved to https://gitlab.com/gitterHQ (from https://gitlab.com/gitlab-org/gitter) and there's no devopscreate label over there. See:

https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/856861152

This is blocking label inference:

Action items

• (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 =>
  • Week-in-review