net: remove redundant ifdef CONFIG_CGROUPS

The cgroup code has been surrounded by ifdef CONFIG_NET_CLS_CGROUP
and CONFIG_NETPRIO_CGROUP.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>