net/sched: remove redundant null check on head

head is previously null checked and so the 2nd null check on head
is redundant and therefore can be removed.

Detected by CoverityScan, CID#1399505 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>