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: https://github.com/nodejs/node/issues/13836
PR-URL: https://github.com/nodejs/node/pull/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-nopendingdep-map.js

@@ -2,7 +2,6 @@
 'use strict';
 
 const common = require('../common');
-const Buffer = require('buffer').Buffer;
 
 process.on('warning', common.mustNotCall('A warning should not be emitted'));
 

test/parallel/test-buffer-pending-deprecation.js

@@ -2,7 +2,6 @@
 'use strict';
 
 const common = require('../common');
-const Buffer = require('buffer').Buffer;
 
 const bufferWarning = 'The Buffer() and new Buffer() constructors are not ' +
                       'recommended for use due to security and usability ' +

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

test/parallel/test-buffer-tojson.js

@@ -2,7 +2,6 @@
 
 require('../common');
 const assert = require('assert');
-const Buffer = require('buffer').Buffer;
 
 {
   assert.strictEqual(JSON.stringify(Buffer.alloc(0)),

test/parallel/test-buffer-zero-fill.js

@@ -2,7 +2,6 @@
 
 require('../common');
 const assert = require('assert');
-const Buffer = require('buffer').Buffer;
 
 const buf1 = Buffer(100);
 const buf2 = new Buffer(100);