configs: drop redundant IPv6 option

Drop the buildroot toolchain IPv6 option from the only config that's got
it since it's now a non-option.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>