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

Since commit 8db4c5be ("netfilter: move socket lookup
infrastructure to nf_socket_ipv{4,6}.c") we no longer need to set
CONFIG_NETFILTER_XT_MATCH_SOCKET in our defconfigs.

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