LibJS: Remove mock String.prototype from tests

Libraries/LibJS/Tests/String.prototype.toLowerCase.js

@@ -1,7 +1,4 @@
 try {
-    // FIXME: Remove once we have the global String object
-    var String = { prototype: Object.getPrototypeOf("") };
-
     assert(String.prototype.toLowerCase.length === 0);
 
     assert("foo".toLowerCase() === "foo");

Libraries/LibJS/Tests/String.prototype.toUpperCase.js

@@ -1,7 +1,4 @@
 try {
-    // FIXME: Remove once we have the global String object
-    var String = { prototype: Object.getPrototypeOf("") };
-
     assert(String.prototype.toUpperCase.length === 0);
 
     assert("foo".toUpperCase() === "FOO");