🔥(backend) remove treebeard form for the document admin

The document change admin page is unusable. The django treebeard library
can change the form used by one provided but this one is really slow.
And it is collapsing the configuration made with the other fields and
readonly fields declared on the DocumentAdmin class. In a first time we
remove the form usage, it seems useless. Later we have to provide more
information on this admin page.

src/backend/core/admin.py

@@ -5,7 +5,6 @@ from django.contrib.auth import admin as auth_admin
 from django.utils.translation import gettext_lazy as _
 
 from treebeard.admin import TreeAdmin
-from treebeard.forms import movenodeform_factory
 
 from . import models
 
@@ -157,7 +156,6 @@ class DocumentAdmin(TreeAdmin):
             },
         ),
     )
-    form = movenodeform_factory(models.Document)
     inlines = (DocumentAccessInline,)
     list_display = (
         "id",