Remove redundant `travel_back`

Since #29860, `travel_back` automatically called at the end of the test.

activerecord/test/cases/dirty_test.rb

@@ -567,8 +567,6 @@ def test_previous_changes
     assert_not_nil pirate.previous_changes["updated_on"][1]
     assert_not pirate.previous_changes.key?("parrot_id")
     assert_not pirate.previous_changes.key?("created_on")
-  ensure
-    travel_back
   end
 
   class Testings < ActiveRecord::Base; end

activesupport/test/metadata/shared_metadata_tests.rb

 # frozen_string_literal: true
 
 module SharedMessageMetadataTests
-  def teardown
-    travel_back
-    super
-  end
-
   def null_serializing?
     false
   end