net: sched: cls_matchall: fix null pointer dereference

Since the head is guaranteed by the check above to be null, the call_rcu
would explode. Remove the previously logically dead code that was made
logically very much alive and kicking.

Fixes: 985538ee ("net/sched: remove redundant null check on head")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>