Remove deployment_frequency_charts feature flag

This commit removes the `deployment_frequency_charts` feature flag.

ee/app/helpers/ee/graph_helper.rb

@@ -6,7 +6,6 @@ module EE
 
     override :should_render_deployment_frequency_charts
     def should_render_deployment_frequency_charts
-      return false unless ::Feature.enabled?(:deployment_frequency_charts, @project, default_enabled: true)
       return false unless @project.feature_available?(:dora4_analytics)
 
       can?(current_user, :read_dora4_analytics, @project)

ee/changelogs/unreleased/nfriend-remove-deployment_frequency_charts-feature-flag.yml

+---
+title: Remove deployment_frequency_charts feature flag
+merge_request: 59289
+author:
+type: removed

ee/config/feature_flags/development/deployment_frequency_charts.yml

----
-name: deployment_frequency_charts
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50885
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/296893
-milestone: '13.8'
-type: development
-group: group::release
-default_enabled: true

ee/spec/helpers/ee/graph_helper_spec.rb

@@ -16,12 +16,6 @@ RSpec.describe EE::GraphHelper do
   end
 
   describe '#should_render_deployment_frequency_charts' do
-    let(:is_flag_enabled) { true }
-
-    before do
-      stub_feature_flags(deployment_frequency_charts: is_flag_enabled)
-    end
-
     shared_examples 'returns true' do
       it { expect(should_render_deployment_frequency_charts).to be(true) }
     end
@@ -38,12 +32,6 @@ RSpec.describe EE::GraphHelper do
       it_behaves_like 'returns false'
     end
 
-    context 'when the feature flag is disabled' do
-      let(:is_flag_enabled) { false }
-
-      it_behaves_like 'returns false'
-    end
-
     context 'when the user does not have permission' do
       let(:is_user_authorized) { false }