Remove the "delete_references" option and functionality

Patch doesn't fix the problem. I believe this is the reason:

+++ b/src/Plugin/Field/FieldWidget/InlineEntityFormMultiple.php
@@ -777,6 +777,9 @@ class InlineEntityFormMultiple extends InlineEntityFormBase implements Container
+      if (!empty($form_values['delete'])) {
+        $entity->delete();
+      }

We should read from field configuration instead of form state.

We also have a whole commented out function for this:

/**
 * Implements hook_entity_delete().
 *
 * Deletes referenced entities if needed.
 *
 * @todo Remove when there's a stable contrib module for this.
 */
function inline_entity_form_entity_delete(EntityInterface $entity) {
  return;

How about removing the entire feature? It no longer feels like something a widget should do, cause that means that an API level delete won't have the same effect.

Another thing that also crossed my mind is the fact that the referenced entity won't be deleted if we delete parent entity. This is somewhat confusing IMO as I'd expect for both cases to behave same way.

Could we do this in a way that would also work on API level? Or is this something that should be done in a separate module that would handle this for all referenced entities?

Separate module that integrates with entity reference, for sure.
Dave Reid already has a D7 contrib for this, though we'll probably want to revisit the approach for D8.

Retitling.

Done.

This is irrelevant now as we removed this feature from the module (see #8).