Remove some unnecessary calls to clear the mail queue

These were not needed at the end of most tests for quite some time,
since rails clears them by default for controller and integration tests,
and also no longer required in system tests after #5691

test/controllers/confirmations_controller_test.rb

@@ -204,8 +204,6 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
     email = ActionMailer::Base.deliveries.last
 
     assert_equal user.email, email.to.first
-
-    ActionMailer::Base.deliveries.clear
   end
 
   def test_confirm_resend_deleted