powerpc/configs/6xx: Drop no longer needed CONFIG_NF_CONNTRACK_PROC_COMPAT

Since commit adf05168 ("netfilter: remove ip_conntrack* sysctl
compat code") we no longer need to set CONFIG_NF_CONNTRACK_PROC_COMPAT
in our defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>