remove __allow_unmapped__ requirement from dataclasses

remove __allow_unmapped__ requirement from dataclasses

Removed the requirement that the ``__allow_unmapped__`` attribute be used
on Declarative Dataclass Mapped class when non-``Mapped[]`` annotations are
detected; previously, an error message that was intended to support legacy
ORM typed mappings would be raised, which additionally did not mention
correct patterns to use with Dataclasses specifically. This error message
is now no longer raised if :meth:`_orm.registry.mapped_as_dataclass` or
:class:`_orm.MappedAsDataclass` is used.

Fixes: #8973
Change-Id: I887afcc2da83dd904444bcb97f31e695b9f8b443