Remove incorrect localVue usage

localVue is required only with use method

spec/frontend/diffs/components/diff_gutter_avatars_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import DiffGutterAvatars from '~/diffs/components/diff_gutter_avatars.vue';
 import discussionsMockData from '../mock_data/diff_discussions';
 
-const localVue = createLocalVue();
 const getDiscussionsMockData = () => [Object.assign({}, discussionsMockData)];
 
 describe('DiffGutterAvatars', () => {
@@ -14,7 +13,6 @@ describe('DiffGutterAvatars', () => {
 
   const createComponent = (props = {}) => {
     wrapper = shallowMount(DiffGutterAvatars, {
-      localVue,
       propsData: {
         ...props,
       },

spec/frontend/diffs/components/edit_button_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import EditButton from '~/diffs/components/edit_button.vue';
 
-const localVue = createLocalVue();
 const editPath = 'test-path';
 
 describe('EditButton', () => {
@@ -9,7 +8,6 @@ describe('EditButton', () => {
 
   const createComponent = (props = {}) => {
     wrapper = shallowMount(EditButton, {
-      localVue,
       propsData: { ...props },
       sync: false,
       attachToDocument: true,

spec/frontend/diffs/components/hidden_files_warning_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import HiddenFilesWarning from '~/diffs/components/hidden_files_warning.vue';
 
-const localVue = createLocalVue();
 const propsData = {
   total: '10',
   visible: 5,
@@ -14,7 +13,6 @@ describe('HiddenFilesWarning', () => {
 
   const createComponent = () => {
     wrapper = shallowMount(HiddenFilesWarning, {
-      localVue,
       sync: false,
       propsData,
     });

spec/frontend/diffs/components/no_changes_spec.js

@@ -13,7 +13,7 @@ describe('Diff no changes empty state', () => {
     const store = createStore();
     extendStore(store);
 
-    vm = shallowMount(localVue.extend(NoChanges), {
+    vm = shallowMount(NoChanges, {
       localVue,
       store,
       propsData: {