Filter attributes we don't map during a load_scalar_attributes

Filter attributes we don't map during a load_scalar_attributes

Fixed bug where a descriptor that is elsewhere a mapped column
or relationship within a hierarchy based on :class:`.AbstractConcreteBase`
would be referred towards during a refresh operation, causing an error
as the attribute is not mapped as a mapper property.
A similar issue can arise for other attributes like the "type" column
added by :class:`.AbstractConcreteBase` if the class fails to include
"concrete=True" in its mapper, however the check here should also
prevent that scenario from causing a problem.

Change-Id: I407b07a3a3e2c374da19fc86ed44b987d595dcfa
Fixes: #4124
(cherry picked from commit fd4289c5829d6498495ac59fe1dccb23b4975281)