Remove descriptor for field name

Unfortunately, adding a proxy for field's name is adding more complexity when using the field. I've just run into an error when trying to update a field's value.

```python
def test_update(self):
        expected = 'bonjour'
        instance = EncryptedTextFieldModelFactory.create()
        instance.pgp_pub_field = expected
        instance.save()
        self.assertEqual(instance.pgp_pub_field, expected)
```

Probably best to have instead a descriptor for the decrypted value and to leave alone the original field.