types: remove unecessary lines

Author: yyx990803

src/core/instance/lifecycle.js

@@ -60,9 +60,6 @@ export function lifecycleMixin (Vue: Class<Component>) {
     if (!prevVnode) {
       // initial render
       vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
-      // no need for the ref nodes after initial patch
-      // this prevents keeping a detached DOM tree in memory (#5851)
-      vm.$options._parentElm = vm.$options._refElm = null
     } else {
       // updates
       vm.$el = vm.__patch__(prevVnode, vnode)