Remove action_monthly_active_users metrics

What does this MR do and why?

Related to #391308 (closed)

We want to remove metrics that are not necessary.

Screenshots or screen recordings

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How to set up and validate locally

To make sure that Service Ping is still generated correctly, enter the rails console and run:

sp = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)

To make sure that the metrics are successfully removed from the payload:

[9] pry(main)> sp.dig('usage_activity_by_stage_monthly', 'create', 'action_monthly_active_users_sfe_edit')
=> nil
[10] pry(main)> sp.dig('usage_activity_by_stage_monthly', 'create', 'action_monthly_active_users_snippet_editor_edit')
=> nil

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.