Remove duplicate DefaultReceiverTest

This was accidentally copy & pasted in this commit [1] as part of #726.
The canonical version is in `test/unit/default_receiver_test.rb`.

The presence of the duplicate version was causing these warnings in the
`test:units` rake task:

    test/unit/default_receiver_test.rb:9: warning: method redefined; discarding old test_mocks_returns_mock
    test/unit/any_instance_receiver_test.rb:44: warning: previous definition of test_mocks_returns_mock was here

[1]: 8da6a53

test/unit/any_instance_receiver_test.rb

@@ -37,13 +37,3 @@ def test_mocks_returns_mocks_for_class_and_its_superclasses
     assert_equal [:mocha, :parent_mocha, :grandparent_mocha], receiver.mocks
   end
 end
-
-class DefaultReceiverTest < Mocha::TestCase
-  include Mocha
-
-  def test_mocks_returns_mock
-    mock = :mocha
-    receiver = DefaultReceiver.new(mock)
-    assert_equal [:mocha], receiver.mocks
-  end
-end