Remove duplicate metric usage_activity_by_stage_monthly.create.merge_requests_users

Problem

usage_activity_by_stage_monthly.create.merge_requests_users metric is a duplicate of redis_hll_counters.source_code.merge_request_action_{monthly, weekly} and it is not using instrumentation classes.

Solution

Remove usage_activity_by_stage_monthly.create.merge_requests_users in favor of redis_hll_counters.source_code.merge_request_action_{monthly, weekly} according to our process: https://docs.gitlab.com/ee/development/service_ping/metrics_lifecycle.html#remove-a-metric

Issue requesting check if metric is used in any dashboards: https://gitlab.com/gitlab-data/analytics/-/issues/15922