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.

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

Author: lpinca

lib/internal/streams/BufferList.js

@@ -61,8 +61,6 @@ module.exports = class BufferList {
   concat(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;