use load_scalar_attributes() for undefer

use load_scalar_attributes() for undefer

At some point, undeferral of attributes started emitting
a full ORM query for the entity, including that subclass columns
would use a JOIN.   Seems to be at least in 1.3 / 1.4 and possibly
earlier.    This JOINs to the superclass table unnecessarily.
Use load_scalar_attributes() here which should handle the whole
thing and emits a more efficient query for joined inheritance.

As this behavior seems to have been throughout 1.3 and 1.4
at least, targeting at 2.0 is likely best.

Fixes: #7463
Change-Id: Ie4bae767747bba0d03fb13eaff579d4bab0b1bc2