[chores] Get rid of GenericField workaround

[nemesifier]

Follow up of #345.

Get rid of the workaround for bug https://code.djangoproject.com/ticket/36295 once this is fixed in Django.

[nemesifier]

@pandafy the django ticket has been fixed: https://code.djangoproject.com/ticket/36295, but I believe the fix is not released yet, once it's fixed we can update the code accordingly.