fix: use vue-cli default source-map

https://github.com/PanJiaChen/vue-element-admin/commit/d4405464ceb53513bff7ec6caf07113623e95383

vue.config.js

@@ -80,12 +80,6 @@ module.exports = {
       })
       .end()
 
-    config
-    // https://webpack.js.org/configuration/devtool/#development
-      .when(process.env.NODE_ENV === 'development',
-        config => config.devtool('cheap-source-map')
-      )
-
     config
       .when(process.env.NODE_ENV !== 'development',
         config => {