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/users_controller_test.rb

@@ -87,8 +87,6 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
 
     # Check the page
     assert_redirected_to :controller => :confirmations, :action => :confirm, :display_name => user.display_name
-
-    ActionMailer::Base.deliveries.clear
   end
 
   def test_create_duplicate_email
@@ -189,8 +187,6 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         perform_enqueued_jobs
       end
     end
-
-    ActionMailer::Base.deliveries.clear
   end
 
   def test_go_public