powerpc/config: Fix IPV6 warning in mpc855_ads

When building this config there's a warning:

  79:warning: override: reassigning to symbol IPV6

Commit 9a1762a4 ("powerpc/8xx: Update mpc885_ads_defconfig to
improve CI") added CONFIG_IPV6=y, but left '# CONFIG_IPV6 is not set'
in.

IPV6 is default y, so remove both to clean up the build.

Fixes: 9a1762a4

 ("powerpc/8xx: Update mpc885_ads_defconfig to improve CI")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210817045407.2445664-2-joel@jms.id.au