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

arch/powerpc/configs/mpc885_ads_defconfig

@@ -21,7 +21,6 @@ CONFIG_INET=y
 CONFIG_IP_MULTICAST=y
 CONFIG_IP_PNP=y
 CONFIG_SYN_COOKIES=y
-# CONFIG_IPV6 is not set
 # CONFIG_FW_LOADER is not set
 CONFIG_MTD=y
 CONFIG_MTD_BLOCK=y
@@ -76,7 +75,6 @@ CONFIG_PERF_EVENTS=y
 CONFIG_MATH_EMULATION=y
 CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y
 CONFIG_STRICT_KERNEL_RWX=y
-CONFIG_IPV6=y
 CONFIG_BPF_JIT=y
 CONFIG_DEBUG_VM_PGTABLE=y
 CONFIG_BDI_SWITCH=y