powerpc/configs/64s: Drop IPV6 which is default y

Since commit de551f2e ("net: Build IPv6 into kernel by default"),
IPV6 is default y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230414132415.821564-3-mpe@ellerman.id.au