refactor: let vue-loader handle <template lang="xxx">

BREAKING CHANGE: vuejs/vue-loader@c954f32

lib/builder/webpack/base.mjs

@@ -1,7 +1,6 @@
 import path from 'path'
 
 import TimeFixPlugin from 'time-fix-plugin'
-import webpack from 'webpack'
 import _ from 'lodash'
 import VueLoader from 'vue-loader'
 import MiniCssExtractPlugin from 'mini-css-extract-plugin'
@@ -119,9 +118,6 @@ export default class WebpackBaseConfig {
         loader: 'vue-loader',
         options: Object.assign({
           productionMode: !this.options.dev,
-          template: {
-            doctype: 'html' // For pug, see https://github.com/vuejs/vue-loader/issues/55
-          },
           transformAssetUrls: {
             video: 'src',
             source: 'src',