stream: delete unused code

In implementation of `stream.Writable`, `writable._write()` is
always called with a callback that is `_writableState.onwrite()`.
And In `afterTransform()`, `ts.writechunk` and `ts.writecb` are
assigned to null.  So, `ts.writecb` is a true value if
`ts.writechunk` isn't null.

PR-URL: #18278
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_transform.js

@@ -180,7 +180,7 @@ Transform.prototype._write = function(chunk, encoding, cb) {
 Transform.prototype._read = function(n) {
   var ts = this._transformState;
 
-  if (ts.writechunk !== null && ts.writecb && !ts.transforming) {
+  if (ts.writechunk !== null && !ts.transforming) {
     ts.transforming = true;
     this._transform(ts.writechunk, ts.writeencoding, ts.afterTransform);
   } else {