unix: remove erroneous asserts

As of commit c53fe81, it's legal for write_queue_size > 0 when the
write_queue itself is empty. Sounds illogical but it means there are
error-state write requests in the write_completed_queue that will touch
up the write_queue_size on the next tick of the event loop.

Remove a few stray asserts that still checked for the old situation.