Remove duplicate metric create.merge_requests_users

What does this MR do and why?

The metric usage_activity_by_stage_monthly.create.merge_requests_users is reduntant now that we have redis_hll_counters.source_code.merge_request_action_{monthly, weekly}.

Therefore, we are removing it.

The potential affected teams have confirmed that the metric can be removed without impacting them.

See: https://gitlab.com/gitlab-data/analytics/-/issues/15922 and #391946 (closed)

Notice that "usage_activity_by_stage_monthly.create.merge_requests_users" is used as a label for a number of Snowplow events. The removal of the label might be part of another MR.

MR acceptance checklist

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

Related to #391946 (closed)