Remove duplicate WeakSet test

This test is duplicated by WeakSet/prototype/add/value-not-object-throw.js

Originally taken from Rick's draft commit for symbols-as-weakmap-keys
tests.

test/built-ins/WeakSet/symbol-disallowed-as-weakset-key.js

@@ -1,14 +0,0 @@
-// Copyright (C) 2013 the V8 project authors. All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-/*---
-esid: sec-weakset.prototype.add
-description: >
-  Symbol may not be used as a WeakSet entry
-features: [Symbol, WeakSet]
----*/
-var weakset = new WeakSet();
-var sym = Symbol();
-
-assert.throws(TypeError, function() {
-  weakset.add(sym);
-});