Cancel polymorphic loading in optimized get

Cancel polymorphic loading in optimized get

Since optimized_get for inheriting mappers writes a simple
SELECT, we need to cancel out any with_polymorphic selectables
that interfere with simple column lookup.  While adaptation is
another option, just removing the with_polymorphic is much
simpler.   The issue is not noticeable unless the ResultProxy
is not allowing "key fallback" column lookups, which will
be the case when this behavior is deprecated.

Fixes: #4718
Change-Id: I8fa2f5c0434b6a681813a92ac71fe12712f5d634