Remove `improved_mr_merged_at_queries` feature flag

app/models/merge_request.rb

@@ -257,11 +257,7 @@ class MergeRequest < ApplicationRecord
   scope :join_project, -> { joins(:target_project) }
   scope :join_metrics, -> do
     query = joins(:metrics)
-
-    if Feature.enabled?(:improved_mr_merged_at_queries, default_enabled: true)
     query = query.where(MergeRequest.arel_table[:target_project_id].eq(MergeRequest::Metrics.arel_table[:target_project_id]))
-    end
-
     query
   end
   scope :references_project, -> { references(:target_project) }

config/feature_flags/development/improved_mr_merged_at_queries.yml

----
-name: improved_mr_merged_at_queries
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39329
-rollout_issue_url: 
-group: group::analytics
-type: development
-default_enabled: true