net/flow_dissector: remove unexist field description

@thoff has moved to struct flow_dissector_key_control.

Fixes: 42aecaa9

 ("net: Get skb hash over flow_keys structure")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>