Remove env_last_deployment_by_finished_at feature flag

Changelog: removed

app/models/environment.rb

@@ -26,7 +26,7 @@ class Environment < ApplicationRecord
   has_many :self_managed_prometheus_alert_events, inverse_of: :environment
   has_many :alert_management_alerts, class_name: 'AlertManagement::Alert', inverse_of: :environment
-  has_one :last_deployment, -> { Feature.enabled?(:env_last_deployment_by_finished_at, default_enabled: :yaml) ? success.ordered : success.distinct_on_environment }, class_name: 'Deployment', inverse_of: :environment
+  has_one :last_deployment, -> { success.ordered }, class_name: 'Deployment', inverse_of: :environment
   has_one :last_visible_deployment, -> { visible.distinct_on_environment }, inverse_of: :environment, class_name: 'Deployment'
   has_one :last_visible_deployable, through: :last_visible_deployment, source: 'deployable', source_type: 'CommitStatus', disable_joins: true
   has_one :last_visible_pipeline, through: :last_visible_deployable, source: 'pipeline', disable_joins: true

config/feature_flags/development/env_last_deployment_by_finished_at.yml

----
-name: env_last_deployment_by_finished_at
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83558
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/357299
-milestone: '14.10'
-type: development
-group: group::release
-default_enabled: true

spec/models/environment_spec.rb

@@ -886,22 +886,10 @@
           is_expected.to eq(deployment)
         end
-        context 'env_last_deployment_by_finished_at feature flag' do
-          it 'when enabled it returns the deployment with the latest finished_at' do
-            stub_feature_flags(env_last_deployment_by_finished_at: true)
-            expect(old_deployment.finished_at < deployment.finished_at).to be_truthy
-
-            is_expected.to eq(deployment)
-          end
-
-          it 'when disabled it returns the deployment with the highest id' do
-            stub_feature_flags(env_last_deployment_by_finished_at: false)
-
-            expect(old_deployment.finished_at < deployment.finished_at).to be_truthy
-
-            is_expected.to eq(old_deployment)
-          end
+        it 'returns the deployment with the latest finished_at' do
+          expect(old_deployment.finished_at < deployment.finished_at).to be_truthy
+          is_expected.to eq(deployment)
         end
       end
     end