remove excessively clever TestOrderedSet.test_order_retention method

The most recent iteration of this test was fundamentally broken. My
colleagues pointed out that it was too clever by half and there's no
need to test the behavior of set, only OrderedSet. They're right. The
other test methods should be sufficient.

Change-Id: Ibe3b531739f56981363fac75f3a86d1a104f3596
Merge "remove excessively clever TestOrderedSet.test_order_retention method"