test: remove unnecessary require('buffer').Buffer

Remove unnecessary imports from:

* test/parallel/test-buffer-nopendingdep-map.js
* test/parallel/test-buffer-pending-deprecation.js
* test/parallel/test-buffer-sharedarraybuffer.js
* test/parallel/test-buffer-slow.js
* test/parallel/test-buffer-tojson.js
* test/parallel/test-buffer-zero-fill.js

Refs: #13836
PR-URL: #13851
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-sharedarraybuffer.js

@@ -4,7 +4,6 @@
 
 require('../common');
 const assert = require('assert');
-const Buffer = require('buffer').Buffer;
 
 const sab = new SharedArrayBuffer(24);
 const arr1 = new Uint16Array(sab);

test/parallel/test-buffer-slow.js

@@ -3,7 +3,6 @@
 require('../common');
 const assert = require('assert');
 const buffer = require('buffer');
-const Buffer = buffer.Buffer;
 const SlowBuffer = buffer.SlowBuffer;
 
 const ones = [1, 1, 1, 1];