Remove test that's not useful

This test was originally added when ActiveSupport::Deprecation was a
Module in 2c690a0. It was testing that
including the module and changing the behavior would not impact the
global deprecation behavior.

When ActiveSupport::Deprecation was changed to be a Class in
71993c6, it started to mutate the
global state, changing the behavior of the singleton. And the test lost
its meaning, it wasn't testing anymore that one Deprecation object
wouldn't impact another.

I didn't add such a test because it's default behavior that changing one
object doesn't impact another.

activesupport/test/deprecation_test.rb

@@ -411,20 +411,6 @@ def setup
     assert_deprecated(/this is the old way/, @deprecator) { klass.new.fubar }
   end
 
-  test "delegating to ActiveSupport::Deprecation" do
-    messages = []
-
-    klass = Class.new do
-      delegate :warn, :behavior=, to: ActiveSupport::Deprecation
-    end
-
-    o = klass.new
-    o.behavior = Proc.new { |message, callstack| messages << message }
-    assert_difference("messages.size") do
-      o.warn("warning")
-    end
-  end
-
   test "overriding deprecated_method_warning" do
     deprecator = deprecator_with_messages