Asserting that we definitely saw the "received a single result" flag
and can now deal with null responses, isn't applicable after a few
recent fixes. New requests are sent out before responses are fully
processed to keep data nodes in a tighter loop (as well as other
relaxed ordering relative to when this assertion was added) so
the flag is not guaranteed to show up as true for lower numbers
of shard requests any longer.
Lets just remove it, it was always best effort and accidental that
this worked for the numbers the test randomizes over.