chore: clean expired code

packages/@vuepress/core/lib/app/util.js

-import Vue from 'vue'
-import { loadComponent } from '@internal/async-component'
-
 /**
  * Inject option to Vue SFC
  * @param {object} options
@@ -38,13 +35,6 @@ export function findPageForPath (pages, path) {
   }
 }
 
-export function registerComponent (key) {
-  return loadComponent(key).then(comp => {
-    Vue.component(key, comp.default)
-    return comp.default
-  })
-}
-
 /**
  * Normalize config.
  * This utility is mainly for plugin developers. For some

packages/@vuepress/core/lib/internal-plugins/routes.js

@@ -14,7 +14,7 @@ module.exports = (options, ctx) => ({
  */
 function importCode () {
   return `
-import { injectComponentOption, registerComponent } from '@app/util'
+import { injectComponentOption } from '@app/util'
 import rootMixins from '@internal/root-mixins'
 import components from '@internal/layout-components'
 import LayoutDistributor from '@app/components/LayoutDistributor.vue'