Remove incorrect localVue usage

localVue is required only with use method

ee/spec/frontend/related_items_tree/components/related_items_tree_app_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import { GlLoadingIcon } from '@gitlab/ui';
 
 import RelatedItemsTreeApp from 'ee/related_items_tree/components/related_items_tree_app.vue';
@@ -19,8 +19,6 @@ import {
   mockParentItem,
 } from '../../../javascripts/related_items_tree/mock_data';
 
-const localVue = createLocalVue();
-
 const mockProjects = getJSONFixture('static/projects.json');
 
 const createComponent = () => {
@@ -29,8 +27,7 @@ const createComponent = () => {
   store.dispatch('setInitialConfig', mockInitialConfig);
   store.dispatch('setInitialParentItem', mockParentItem);
 
-  return shallowMount(localVue.extend(RelatedItemsTreeApp), {
-    localVue,
+  return shallowMount(RelatedItemsTreeApp, {
     store,
     sync: false,
   });

ee/spec/frontend/related_items_tree/components/related_items_tree_header_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import { GlButton } from '@gitlab/ui';
 
 import RelatedItemsTreeHeader from 'ee/related_items_tree/components/related_items_tree_header.vue';
@@ -15,7 +15,6 @@ import {
 
 const createComponent = ({ slots } = {}) => {
   const store = createDefaultStore();
-  const localVue = createLocalVue();
   const children = epicUtils.processQueryResponse(mockQueryResponse.data.group);
 
   store.dispatch('setInitialParentItem', mockParentItem);
@@ -33,7 +32,6 @@ const createComponent = ({ slots } = {}) => {
   return shallowMount(RelatedItemsTreeHeader, {
     attachToDocument: true,
     sync: false,
-    localVue,
     store,
     slots,
   });

ee/spec/frontend/related_items_tree/components/tree_item_body_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import { GlButton, GlLink } from '@gitlab/ui';
 
 import ItemWeight from 'ee/boards/components/issue_card_weight.vue';
@@ -28,8 +28,6 @@ const mockItem = Object.assign({}, mockIssue1, {
   assignees: epicUtils.extractIssueAssignees(mockIssue1.assignees),
 });
 
-const localVue = createLocalVue();
-
 const createComponent = (parentItem = mockParentItem, item = mockItem) => {
   const store = createDefaultStore();
   const children = epicUtils.processQueryResponse(mockQueryResponse.data.group);
@@ -49,7 +47,6 @@ const createComponent = (parentItem = mockParentItem, item = mockItem) => {
   return shallowMount(TreeItemBody, {
     attachToDocument: true,
     sync: false,
-    localVue,
     store,
     propsData: {
       parentItem,

ee/spec/frontend/related_items_tree/components/tree_item_spec.js

-import { shallowMount, createLocalVue } from '@vue/test-utils';
+import { shallowMount } from '@vue/test-utils';
 import { GlButton, GlLoadingIcon } from '@gitlab/ui';
 
 import TreeItem from 'ee/related_items_tree/components/tree_item.vue';
@@ -23,8 +23,6 @@ const mockItem = Object.assign({}, mockEpic1, {
   pathIdSeparator: PathIdSeparator.Epic,
 });
 
-const localVue = createLocalVue();
-
 const createComponent = (parentItem = mockParentItem, item = mockItem) => {
   const store = createDefaultStore();
   const children = epicUtils.processQueryResponse(mockQueryResponse.data.group);
@@ -45,8 +43,7 @@ const createComponent = (parentItem = mockParentItem, item = mockItem) => {
     isSubItem: true,
   });
 
-  return shallowMount(localVue.extend(TreeItem), {
-    localVue,
+  return shallowMount(TreeItem, {
     store,
     stubs: {
       'tree-root': TreeRoot,