Remove refcount check

This may be 1 if the `LOAD_FAST` is optimized to a `LOAD_FAST_BORROW`. It's
not clear that this is testing anything useful, so remove it.

Author: mpage

Lib/test/test_ctypes/test_memfunctions.py

@@ -60,9 +60,6 @@ def test_cast(self):
     @support.refcount_test
     def test_string_at(self):
         s = string_at(b"foo bar")
-        # XXX The following may be wrong, depending on how Python
-        # manages string instances
-        self.assertEqual(2, sys.getrefcount(s))
         self.assertTrue(s, "foo bar")
 
         self.assertEqual(string_at(b"foo bar", 7), b"foo bar")