[DX] Form API: Be able to tell system_settings_form() to ignore some form elements, without having to use a custom submit handler

Description of the need

I've got a form with two submit handlers. It's using system_settings_form and a custom one. There's one form element that should not be saved to config. The issue is that system_settings_form is saving them all. According to https://api.backdropcms.org/api/backdrop/core%21modules%21system%21syste... you can add additional config files but there doesn't seem to be a way to ignore a form element entirely, and not create a second settings file.

Follow-up from https://github.com/backdrop/backdrop-issues/issues/675