fs: inline typeof check

PR-URL: nodejs#24390
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/fs/streams.js

@@ -204,8 +204,7 @@ ReadStream.prototype._read = function(n) {
 };
 
 ReadStream.prototype._destroy = function(err, cb) {
-  const isOpen = typeof this.fd !== 'number';
-  if (isOpen) {
+  if (typeof this.fd !== 'number') {
     this.once('open', closeFsStream.bind(null, this, cb, err));
     return;
   }