Removals: strings for join(), loader_options().

Removals: strings for join(), loader_options().

* The :meth:`_orm.Query.join` method no longer accepts strings for
relationship names; the long-documented approach of using
``Class.attrname`` for join targets is now standard.

* Loader options no longer accept strings for attribute names.  The
long-documented approach of using ``Class.attrname`` for loader option
targets is now standard.

It is hoped that a subsequent commit can refactor loader
options to no longer need "UnboundLoad" for most cases.

Change-Id: If4629882c40523dccbf4459256bf540fb468b618
References: #6986