Don't warn for mixin-based __table_args__, __mapper_args__ declared_attr

Don't warn for mixin-based __table_args__, __mapper_args__ declared_attr

Removed a warning that would be emitted when calling upon
``__table_args__``, ``__mapper_args__`` as named with a ``@declared_attr``
method, when called from a non-mapped declarative mixin.  Calling these
directly is documented as the approach to use when one is overidding one
of these methods on a mapped class.  The warning still emits for regular
attribute names.

Change-Id: Iae7ed0bd625a2c163c910aa777cef4779128580a
Fixes: #4221
(cherry picked from commit 37955a52995cdbb66a9d5835c20ee58fb98ddffc)