stream: remove vestiges of previous _transform API

lib/_stream_transform.js

@@ -152,7 +152,7 @@ Transform.prototype.push = function(chunk) {
 // Call `cb(err)` when you are done with this chunk.  If you pass
 // an error, then that'll put the hurt on the whole operation.  If you
 // never call cb(), then you'll never get another chunk.
-Transform.prototype._transform = function(chunk, output, cb) {
+Transform.prototype._transform = function(chunk, encoding, cb) {
   throw new Error('not implemented');
 };
 
@@ -171,7 +171,7 @@ Transform.prototype._write = function(chunk, encoding, cb) {
 };
 
 // Doesn't matter what the args are here.
-// the output and callback functions passed to _transform do all the work.
+// _transform does all the work.
 // That we got here means that the readable side wants more data.
 Transform.prototype._read = function(n) {
   var ts = this._transformState;