Remove the "/cc" quick action
As https://gitlab.com/gitlab-org/gitlab/-/issues/16475 and https://gitlab.com/gitlab-org/gitlab/-/merge_requests/94096 state, the `/cc` quick action doesn't actually do anything. Until we add a benefit to it, we should remove it as a quick action item. /cc @msedlakjakubowski @nickleonard @gweaver
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