Revert "fix: work around npm6/postcss8 hoisting issue (#6358)"

This reverts commit 27b4263.

Author: haoqunjiang

packages/@vue/cli-service/bin/postinstall.js

@@ -1,6 +1,3 @@
-const fs = require('fs-extra')
-const path = require('path')
-
 module.exports = (api, options) => {
   api.render('./template', {
     doesCompile: api.hasPlugin('babel') || api.hasPlugin('typescript')
@@ -77,8 +74,4 @@ module.exports = (api, options) => {
   if (options.configs) {
     api.extendPackage(options.configs)
   }
-
-  if (fs.existsSync(path.resolve(__dirname, './hotfix.js'))) {
-    require('./hotfix')(api, options, options)
-  }
 }

packages/@vue/cli-service/generator/hotfix-npm6only.js

@@ -7,9 +7,6 @@
   "bin": {
     "vue-cli-service": "bin/vue-cli-service.js"
   },
-  "scripts": {
-    "postinstall": "node bin/postinstall.js"
-  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/vuejs/vue-cli.git",