Remove Slack channel for the "infrastructure" group

Why is this change being made?

In the case of failing dev pipelines, if an incident gets the Infrastructure label set, then the infrastructure group is detected, and we send the notification to the Slack channel defined for the group (i.e. #infrastructure-lounge before this MR).

Thee thing is that we shouldn't post bot message in #infrastructure-lounge, so in this MR, we just remove the Slack channel from the group definition, so that the default Slack channel for failing dev pipeline incidents will be used (i.e. #master-broken-mirrors).

That was reported by @marin at https://gitlab.slack.com/archives/C04RC5QTGBT/p1714637448630389.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Remove Slack channel for the "infrastructure" group

---