test: remove require('buffer') from 4 buffer tests

Remove superfluous import from:

* test/parallel/test-buffer-alloc.js
* test/parallel/test-buffer-arraybuffer.js
* test/parallel/test-buffer-badhex.js
* test/parallel/test-buffer-bytelength.js

Refs: https://github.com/nodejs/node/issues/13836
PR-URL: https://github.com/nodejs/node/pull/13855
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-arraybuffer.js

@@ -3,7 +3,6 @@
 require('../common');
 const assert = require('assert');
 
-const Buffer = require('buffer').Buffer;
 const LENGTH = 16;
 
 const ab = new ArrayBuffer(LENGTH);