Remove an overly-aggressive assertion.

While it is true that the new vmspace passed to vmspace_switch_aio
will always have a valid reference due to the AIO job or the extra
reference on the original vmspace in the worker thread, it is not true
that the old vmspace being switched away from will have more than one
reference.

Specifically, when a process with queued AIO jobs exits, the exit hook
in aio_proc_rundown will only ensure that all of the AIO jobs have
completed or been cancelled. However, the last AIO job might have
completed and woken up the exiting process before the worker thread
servicing that job has switched back to its original vmspace. In that
case, the process might finish exiting dropping its reference to the
vmspace before the worker thread resulting in the worker thread
dropping the last reference.

Reported by: np
Reviewed by: alc, markj, np, imp
MFC after: 2 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D20542