[Build] optimize config (#1167)

build/webpack.doc.js

@@ -9,7 +9,6 @@ module.exports = Object.assign(config, {
     path: path.join(__dirname, '../docs/dist'),
     publicPath: 'https://img.yzcdn.cn/zanui/vant/',
     filename: '[name].[hash:8].js',
-    umdNamedDefine: true,
     chunkFilename: 'async_[name].[chunkhash:8].js'
   }
 });