Remove unused dev option from babel-loader (#6624)

packages/next/build/webpack-config.ts

@@ -18,7 +18,7 @@ export default function getBaseWebpackConfig (dir: string, {dev = false, isServe
   const defaultLoaders = {
     babel: {
       loader: 'next-babel-loader',
-      options: { dev, isServer, cwd: dir }
+      options: { isServer, cwd: dir }
     },
     // Backwards compat
     hotSelfAccept: {

packages/next/build/webpack/loaders/next-babel-loader.js

@@ -10,19 +10,17 @@ module.exports = babelLoader.custom(babel => {
   return {
     customOptions (opts) {
       const custom = {
-        isServer: opts.isServer,
-        dev: opts.dev
+        isServer: opts.isServer
       }
       const loader = Object.assign({
         cacheCompression: false,
         cacheDirectory: true
       }, opts)
       delete loader.isServer
-      delete loader.dev
 
       return { loader, custom }
     },
-    config (cfg, { source, customOptions: { isServer, dev } }) {
+    config (cfg, { source, customOptions: { isServer } }) {
       const options = Object.assign({}, cfg.options)
       if (cfg.hasFilesystemConfig()) {
         for (const file of [cfg.babelrc, cfg.config]) {