Removes potentially incorrect, and slow fallback

Deployment_platform is relatively expensive and calling this after the
fact means that this may not be the cluster that was deployed to.

Correspondingly reduce the leeway given in the related N+1 spec

app/models/deployment_metrics.rb

@@ -44,18 +44,7 @@ class DeploymentMetrics
     end
   end
 
-  # TODO remove fallback case to deployment_platform_cluster.
-  # Otherwise we will continue to pay the performance penalty described in
-  # https://gitlab.com/gitlab-org/gitlab-ce/issues/63475
-  #
-  # Removal issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/64105
   def cluster_prometheus
-    cluster_with_fallback = cluster || deployment_platform_cluster
-
-    cluster_with_fallback.application_prometheus if cluster_with_fallback&.application_prometheus_available?
-  end
-
-  def deployment_platform_cluster
-    deployment.environment.deployment_platform&.cluster
+    cluster.application_prometheus if cluster&.application_prometheus_available?
   end
 end

changelogs/unreleased/remove_deployment_metrics_deployment_platform_fallback.yml

+---
+title: Remove incorrect fallback when determining which cluster to use when retrieving
+  MR performance metrics
+merge_request: 31126
+author:
+type: changed

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -885,10 +885,9 @@ describe Projects::MergeRequestsController do
         environment2 = create(:environment, project: forked)
         create(:deployment, :succeed, environment: environment2, sha: sha, ref: 'master', deployable: build)
 
-        # TODO address the last 11 queries
+        # TODO address the last 5 queries
         # See https://gitlab.com/gitlab-org/gitlab-ce/issues/63952 (5 queries)
-        # And https://gitlab.com/gitlab-org/gitlab-ce/issues/64105 (6 queries)
-        leeway = 11
+        leeway = 5
         expect { get_ci_environments_status }.not_to exceed_all_query_limit(control_count + leeway)
       end
     end

spec/models/deployment_metrics_spec.rb

@@ -49,18 +49,6 @@ describe DeploymentMetrics do
 
         it { is_expected.to be_truthy }
       end
-
-      context 'fallback deployment platform' do
-        let(:cluster) { create(:cluster, :provided_by_user, environment_scope: '*', projects: [deployment.project]) }
-        let!(:prometheus) { create(:clusters_applications_prometheus, :installed, cluster: cluster) }
-
-        before do
-          expect(deployment.project).to receive(:deployment_platform).and_return(cluster.platform)
-          expect(cluster.application_prometheus).to receive(:can_query?).and_return(true)
-        end
-
-        it { is_expected.to be_truthy }
-      end
     end
   end