don't set cache header on serviceWorker.js

build/generate_asset_map.js

@@ -19,8 +19,6 @@ function kv(f) {
 module.exports = function() {
   const files = fs.readdirSync(path.join(__dirname, '..', 'assets'));
   const code = `module.exports = {
-    "package.json": require('../package.json'),
-    "serviceWorker.js" : require('../app/serviceWorker.js'),
     ${files.map(kv).join(',\n')}
   };`;
   return {