Remove catching of AttributeError in DescriptorWrapper

This catch was introduced in #367, but subsequent discussion
in #370, #381 and #382 suggests that it wasn't the proper solution.

Now that `AbstractModelTrackerTests` is re-enabled and passing,
it becomes clear that `AttributeError` is no longer raised during
unit testing when using a recent Django version. Therefore I think
it is safe to remove the `try`/`except`.

model_utils/tracker.py

@@ -50,10 +50,7 @@ def __get__(self, instance, owner):
         if instance is None:
             return self
         was_deferred = self.field_name in instance.get_deferred_fields()
-        try:
-            value = self.descriptor.__get__(instance, owner)
-        except AttributeError:
-            value = self.descriptor
+        value = self.descriptor.__get__(instance, owner)
         if was_deferred:
             tracker_instance = getattr(instance, self.tracker_attname)
             tracker_instance.saved_data[self.field_name] = lightweight_deepcopy(value)