Remove docs for setting up post_delete signals on custom document models

This is no longer required as of #3541

docs/advanced_topics/documents/custom_document_model.rst

@@ -10,9 +10,6 @@ You need to complete the following steps in your project to do this:
  - Create a new document model that inherits from
    ``wagtail.wagtaildocs.models.AbstractDocument``. This is where you would
    add additional fields.
- - Add a signal handler to duplicate the post-delete behaviour from
-   ``wagtaildocs.signal_handlers.post_delete_document_file_cleanup```.
-   This ensures the uploaded file is cleaned up when a document is deleted.
  - Point ``WAGTAILDOCS_DOCUMENT_MODEL`` to the new model.
 
 Here's an example:
@@ -20,9 +17,6 @@ Here's an example:
 .. code-block:: python
 
     # models.py
-    from django.db.models.signals import post_delete
-    from django.dispatch import receiver
-
     from wagtail.wagtaildocs.models import Document, AbstractDocument
 
     class CustomDocument(AbstractDocument):
@@ -40,13 +34,6 @@ Here's an example:
             'source',
         )
 
-    # Add the signal handler.
-    # The sender kwarg should match up with your custom document model name.
-    @receiver(post_delete, sender=CustomDocument)
-    def post_delete_document_file_cleanup(sender, instance, **kwargs):
-        # See wagtaildocs.signal_handlers.post_delete_document_file_cleanup
-        instance.file.delete(False)
-
 .. note::
 
     Fields defined on a custom document model must either be set as non-required