Remove fields date_created and date_modified

[ron8mcr]

These fields are redundant for logical deletion.
Model itself named LogicalDeleteModel and seeing
this fields inside it are surprising.

Also, there are a few apps that provide similarly
functionality:

• https://github.com/django-extensions/django-extensions
  It has django_extensions.db.models.TimeStampedModel
• https://github.com/carljm/django-model-utils
  It has model_utils.models.TimeStampedModel

So if somebody started to use one of this app or some another,
and then wants to start using pinax-models with logical
delete feature, he will get confusing by duplicating fields

[jayvdb]

django-model-utils also includes https://django-model-utils.readthedocs.io/en/latest/models.html#softdeletablemodel , which provides soft-deletion. If you only want some fields, and not all, maybe better to just use that package.