Remove unused endpoint to update a group's reporting settings

What does this MR do and why?

Implements part of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/73

After merging !93336 (merged), a group's reporting settings are now updated from the frontend using the update group API endpoint instead of the endpoint introduced in !89996 (merged).

This MR removes the now unused endpoint.

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