Remove FF new_pipeline_form_prefilled_vars

It was enabled by default

app/assets/javascripts/pipeline_new/components/pipeline_new_form.vue

@@ -251,10 +251,6 @@ export default {
       return index < this.variables.length - 1;
     },
     fetchConfigVariables(refValue) {
-      if (!gon?.features?.newPipelineFormPrefilledVars) {
-        return Promise.resolve({ params: {}, descriptions: {} });
-      }
-
       this.isLoading = true;
 
       return backOff((next, stop) => {

app/controllers/projects/pipelines_controller.rb

@@ -17,7 +17,6 @@ class Projects::PipelinesController < Projects::ApplicationController
     push_frontend_feature_flag(:graphql_pipeline_header, project, type: :development, default_enabled: false)
     push_frontend_feature_flag(:graphql_pipeline_details, project, type: :development, default_enabled: :yaml)
     push_frontend_feature_flag(:graphql_pipeline_details_users, current_user, type: :development, default_enabled: :yaml)
-    push_frontend_feature_flag(:new_pipeline_form_prefilled_vars, project, type: :development, default_enabled: true)
   end
   before_action :ensure_pipeline, only: [:show]
   before_action :push_experiment_to_gon, only: :index, if: :html_request?

config/feature_flags/development/new_pipeline_form_prefilled_vars.yml

----
-name: new_pipeline_form_prefilled_vars
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44120
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/263276
-milestone: '13.5'
-type: development
-group: group::continuous integration
-default_enabled: true

spec/frontend/pipeline_new/components/pipeline_new_form_spec.js

@@ -225,23 +225,11 @@ describe('Pipeline New Form', () => {
     });
   });
 
-  describe('when feature flag new_pipeline_form_prefilled_vars is enabled', () => {
-    let origGon;
-
+  describe('when yml defines a variable', () => {
     const mockYmlKey = 'yml_var';
     const mockYmlValue = 'yml_var_val';
     const mockYmlDesc = 'A var from yml.';
 
-    beforeAll(() => {
-      origGon = window.gon;
-      window.gon = { features: { newPipelineFormPrefilledVars: true } };
-    });
-
-    afterAll(() => {
-      window.gon = origGon;
-    });
-
-    describe('loading state', () => {
     it('loading icon is shown when content is requested and hidden when received', async () => {
       createComponent('', mockParams, mount);
 
@@ -258,9 +246,8 @@ describe('Pipeline New Form', () => {
 
       expect(findLoadingIcon().exists()).toBe(false);
     });
-    });
 
-    describe('when yml defines a variable with description', () => {
+    describe('with description', () => {
       beforeEach(async () => {
         createComponent('', mockParams, mount);
 
@@ -302,7 +289,7 @@ describe('Pipeline New Form', () => {
       });
     });
 
-    describe('when yml defines a variable without description', () => {
+    describe('without description', () => {
       beforeEach(async () => {
         createComponent('', mockParams, mount);