net: Remove register_net_sysctl_table

All of the users have been converted to use registera_net_sysctl so we
no longer need register_net_sysctl.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>