remove unused createComponentWithMixin from vue spec helpers

spec/javascripts/helpers/vue_mount_component_helper.js

-import Vue from 'vue';
-
-const mountComponent = (Component, props = {}, el = null) => new Component({
+const mountComponent = (Component, props = {}, el = null) =>
+  new Component({
     propsData: props,
-}).$mount(el);
+  }).$mount(el);
 
-export const createComponentWithStore = (Component, store, propsData = {}) => new Component({
+export const createComponentWithStore = (Component, store, propsData = {}) =>
+  new Component({
     store,
     propsData,
-});
-
-export const createComponentWithMixin = (mixins = [], state = {}, props = {}, template = '<div></div>') => {
-  const Component = Vue.extend({
-    template,
-    mixins,
-    data() {
-      return props;
-    },
   });
 
-  return mountComponent(Component, props);
-};
-
 export const mountComponentWithStore = (Component, { el, props, store }) =>
   new Component({
     store,
-    propsData: props || { },
+    propsData: props || {},
   }).$mount(el);
 
 export default mountComponent;