stream: inline and simplify onwritedrain

Inline and simplify onwritedrain. Also remove comment that seems to be
outdated/invalid.

PR-URL: #29037
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/_stream_writable.js

@@ -478,21 +478,13 @@ function onwrite(stream, er) {
 }
 
 function afterWrite(stream, state, finished, cb) {
-  if (!finished)
-    onwriteDrain(stream, state);
-  state.pendingcb--;
-  cb();
-  finishMaybe(stream, state);
-}
-
-// Must force callback to be called on nextTick, so that we don't
-// emit 'drain' before the write() consumer gets the 'false' return
-// value, and has a chance to attach a 'drain' listener.
-function onwriteDrain(stream, state) {
-  if (state.length === 0 && state.needDrain) {
+  if (!finished && state.length === 0 && state.needDrain) {
     state.needDrain = false;
     stream.emit('drain');
   }
+  state.pendingcb--;
+  cb();
+  finishMaybe(stream, state);
 }
 
 // If there's something in the buffer waiting, then process it