Removed fixed object_id koan

This koan is not as important now that Ruby 2 has changed all the
object ids.

src/about_objects.rb

@@ -30,12 +30,6 @@ class AboutObjects < EdgeCase::Koan
     assert_equal __(true), obj.object_id != another_obj.object_id
   end
 
-  def test_some_system_objects_always_have_the_same_id
-    assert_equal __(0), false.object_id
-    assert_equal __(2), true.object_id
-    assert_equal __(4), nil.object_id
-  end
-
   def test_small_integers_have_fixed_ids
     assert_equal __(1), 0.object_id
     assert_equal __(3), 1.object_id