[modules] Remove outdated TODO in module namespace objects test

As of https://github.com/tc39/ecma262/commit/13906140a, the spec
now returns true when [[SetPrototypeOf]] is invoked with null
on a module namespace object.

R=neis@chromium.org
BUG=v8:1569

Review-Url: https://codereview.chromium.org/2557923004
Cr-Commit-Position: refs/heads/master@{#41575}

test/mjsunit/modules-namespace1.js

@@ -18,7 +18,6 @@ assertTrue(Reflect.preventExtensions(foo));
 assertThrows(() => Reflect.apply(foo, {}, []));
 assertThrows(() => Reflect.construct(foo, {}, []));
 assertSame(null, Reflect.getPrototypeOf(foo));
-// TODO(neis): The next one should be False.
 assertTrue(Reflect.setPrototypeOf(foo, null));
 assertFalse(Reflect.setPrototypeOf(foo, {}));
 assertSame(null, Reflect.getPrototypeOf(foo));