[NET]: Remove unneeded ifdefs from sysctl_net_core.c

This file is already compiled out when the SYSCTL=n, so
these ifdefs, that enclose the whole file, can be removed.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>