Drop stream "shielding" support which was originally added to sidestep
a cancelled call to ``.receive()``

In the original api design a stream instance was returned directly from
a call to ``Portal.run()`` and thus there was no "exit phase" to handle
cancellations and errors which would trigger implicit closure. Now that
we have said enter/exit semantics with ``Portal.open_stream_from()`` and
``Context.open_stream()`` we can drop this implicit (and arguably
confusing) behavior.