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](f683583)
  2. [django-webpack-loader](#2492))

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

Author: haoqunjiang

packages/@vue/cli-plugin-unit-mocha/index.js

@@ -15,12 +15,6 @@ module.exports = api => {
             options.optimizeSSR = false
             return options
           })
-
-      const isLegacyBundle = process.env.VUE_CLI_MODERN_MODE && !process.env.VUE_CLI_MODERN_BUILD
-      // mocha-webpack cannot deal with the query parameter in filename
-      webpackConfig
-        .output
-        .filename(isLegacyBundle ? '[name]-legacy.js?' : '[name].js')
     }
   })
 

packages/@vue/cli-service/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