Stop triggering autosave on delete forms

We can simplify the current solution:

• ContentEntityDeleteForm implements EntityFormInterface, so that check is not necessary.
• $form_object will always be an instance of EntityFormInterface, this is an assumption we can make so we don't need the instanceof check

I'll rebase and commit to the MR.

Merged. Thank you. This will be part of the 1.6 release.

Automatically closed - issue fixed for 2 weeks with no activity.