stream: remove unreachable code

To avoid a function call `BufferList.prototype.concat()` is not called
when there is only a buffer in the list. That buffer is instead
accessed directly.

Backport-PR-URL: #19483
PR-URL: #18239
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/internal/streams/BufferList.js

@@ -58,8 +58,6 @@ BufferList.prototype.join = function(s) {
 BufferList.prototype.concat = function(n) {
   if (this.length === 0)
     return Buffer.alloc(0);
-  if (this.length === 1)
-    return this.head.data;
   const ret = Buffer.allocUnsafe(n >>> 0);
   var p = this.head;
   var i = 0;