Remove errant assertion from unit of work

Remove errant assertion from unit of work

Fixed ORM unit of work regression where an errant "assert primary_key"
statement interferes with primary key generation sequences that don't
actually consider the columns in the table to use a real primary key
constraint, instead using :paramref:`_orm.mapper.primary_key` to establish
certain columns as "primary".

Also remove errant "identity" requirement which does not seem to
represent any current backend and is applied to
test/sql/test_defaults.py->AutoIncrementTest, but these tests work
on all backends.

Fixes: #5867
Change-Id: I4502ca5079d824d7b4d055194947aa1a00effde7