Remove feature flag for env_vars_resource_group

Changelog: removed

config/feature_flags/development/env_vars_resource_group.yml

----
-name: env_vars_resource_group
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67876
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339024
-milestone: '14.3'
-type: development
-group: group::release
-default_enabled: false

lib/gitlab/ci/pipeline/seed/processable/resource_group.rb

@@ -35,7 +35,7 @@ module Gitlab
             def variables
               processable.simple_variables.tap do |variables|
                 # Adding persisted environment variables
-                if Feature.enabled?(:env_vars_resource_group) && processable.persisted_environment.present?
+                if processable.persisted_environment.present?
                   variables.concat(processable.persisted_environment.predefined_variables)
                 end
               end

spec/services/ci/create_pipeline_service_spec.rb

@@ -1041,22 +1041,6 @@ RSpec.describe Ci::CreatePipelineService do
 
         expect(execute_service.payload).to be_created_successfully
       end
-
-      context 'when the env_vars_resource_group feature flag is disabled' do
-        before do
-          stub_feature_flags(env_vars_resource_group: false)
-        end
-
-        it 'does not create a resource group because its key contains an invalid character' do
-          result = execute_service.payload
-          deploy_job = result.builds.find_by_name!(:review_app)
-          stop_job = result.builds.find_by_name!(:stop_review_app)
-          expect(result).to be_persisted
-          expect(deploy_job.resource_group).to be_nil
-          expect(stop_job.resource_group).to be_nil
-          expect(project.resource_groups.count).to eq(0)
-        end
-      end
     end
 
     context 'with timeout' do