Remove invalid assertion

The removed assertion consistently passes not because the invoked
`close` method internally throws a `TypeError` but because the `close`
method does not exist. The ES6 specification does not define a `close`
method on the GeneratorPrototype, so this test is a tautology.

BUG=None
LOG=N
R=arv

Review URL: https://codereview.chromium.org/1046963002

Cr-Commit-Position: refs/heads/master@{#27567}

test/mjsunit/es6/generators-objects.js

@@ -84,7 +84,6 @@ function TestGeneratorObjectMethods() {
     assertThrows(function() { iter.next.call(non_generator); }, TypeError);
     assertThrows(function() { iter.next.call(non_generator, 1); }, TypeError);
     assertThrows(function() { iter.throw.call(non_generator, 1); }, TypeError);
-    assertThrows(function() { iter.close.call(non_generator); }, TypeError);
   }
 
   TestNonGenerator(1);