fix: revert file name hashing in dev mode

closes #2492

The change was intended to mitigate the problem that Safari keeps caching
dev bundles.

But it caused several unintended bugs:
1. [Hashes in file names with HMR enabled might cause memory leaks](webpack/webpack-dev-server#377 (comment))
2. Moving the hash to file query also breaks many webpack loaders:
  1. [mocha-webpack](vuejs/vue-cli@f683583)
  2. [django-webpack-loader](vuejs/vue-cli#2492))

So till we find a better way to solve this issue, hashes in dev bundles
should be removed.

lib/config/base.js

@@ -33,7 +33,7 @@ module.exports = (api, options) => {
         .end()
       .output
         .path(api.resolve(options.outputDir))
-        .filename(isLegacyBundle ? '[name]-legacy.js?[hash:8]' : '[name].js?[hash:8]')
+        .filename(isLegacyBundle ? '[name]-legacy.js' : '[name].js')
         .publicPath(options.baseUrl)
 
     webpackConfig.resolve