Remove obsolete assert_not_deprecated

Follow-up to e9bc620 and
3ec6297.

`assert_not_deprecated` without any arguments will test against
`ActiveSupport::Deprecation.instance`, whereas `Enumerable#sum` would
use `ActiveSupport.deprecator`.  However, the `Enumerable#sum` override
was removed in 3ec6297, so we can
simply remove `assert_not_deprecated`.

Author: jonathanhefner

activesupport/test/core_ext/enumerable_test.rb

@@ -205,9 +205,7 @@ def test_array_sums
     end
     assert_equal SummablePayment.new(20), payments.sum(SummablePayment.new(0)) { |p| p }
 
-    assert_not_deprecated do
-      assert_equal Money.new(3), [Money.new(1), Money.new(2)].sum
-    end
+    assert_equal Money.new(3), [Money.new(1), Money.new(2)].sum
 
     sum = [3, 5.quo(1)].sum
     assert_typed_equal(8, sum, Rational)