Ensure app/mailers is removed when skipping action-mailer

Closes #26145

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -324,6 +324,7 @@ def delete_action_mailer_files_skipping_action_mailer
           remove_file "app/mailers/application_mailer.rb"
           remove_file "app/views/layouts/mailer.html.erb"
           remove_file "app/views/layouts/mailer.text.erb"
+          remove_dir "app/mailers"
         end
       end
 

railties/test/generators/app_generator_test.rb

@@ -380,6 +380,7 @@ def test_generator_if_skip_action_mailer_is_given
     assert_file "config/environments/production.rb" do |content|
       assert_no_match(/config\.action_mailer/, content)
     end
+    assert_no_directory "app/mailers"
   end
 
   def test_generator_has_assets_gems