Remove feature flag usage_activity_by_stage

After the batch counters & a 3 months of indexing improvements, we
have have now zero broken counters in gitlab.com scale

ee/lib/ee/gitlab/usage_data.rb

@@ -37,8 +37,6 @@ module EE
 
         override :uncached_data
         def uncached_data
-          return super if ::Feature.disabled?(:usage_activity_by_stage, default_enabled: true)
-
           time_period = { created_at: 28.days.ago..Time.current }
           usage_activity_by_stage_monthly = usage_activity_by_stage(:usage_activity_by_stage_monthly, time_period)
           super.merge(usage_activity_by_stage).merge(usage_activity_by_stage_monthly)

ee/spec/lib/ee/gitlab/usage_data_spec.rb

@@ -265,18 +265,7 @@ describe Gitlab::UsageData do
       end
 
       describe '.uncached_data' do
-        context 'when the :usage_activity_by_stage feature is not enabled' do
-          before do
-            stub_feature_flags(usage_activity_by_stage: false)
-          end
-
-          it "does not include usage_activity_by_stage data" do
-            expect(described_class.uncached_data).not_to include(:usage_activity_by_stage)
-            expect(described_class.uncached_data).not_to include(:usage_activity_by_stage_monthly)
-          end
-        end
-
-        context 'when the :usage_activity_by_stage feature is enabled' do
+        describe '.usage_activity_by_stage' do
           it 'includes usage_activity_by_stage data' do
             expect(described_class.uncached_data).to include(:usage_activity_by_stage)
             expect(described_class.uncached_data).to include(:usage_activity_by_stage_monthly)