remove "undefer_pks" as a strategy option

remove "undefer_pks" as a strategy option

The behavior of :func:`_orm.defer` regarding primary key and "polymorphic
discriminator" columns is revised such that these columns are no longer
deferrable, either explicitly or when using a wildcard such as
``defer('*')``. Previously, a wildcard deferral would not load
PK/polymorphic columns which led to errors in all cases, as the ORM relies
upon these columns to produce object identities. The behavior of explicit
deferral of primary key columns is unchanged as these deferrals already
were implicitly ignored.

Fixes: #7495
Change-Id: I76d9252426e86619bc142667670a3df75b4f5f6a