support/config-fragments: br-powerpc-internal-full: remove old option

support/config-fragments: br-powerpc-internal-full: remove old option

BR2_TOOLCHAIN_BUILDROOT_INET_IPV6 has been removed with the commit [1].

Since this option is still in br-powerpc-internal-full config-fragment,
the powerpc configuration is droped by autobuild-run script:

WARN: toolchain can't be used
      Missing: BR2_TOOLCHAIN_BUILDROOT_INET_IPV6=y
[Sat, 06 Jan 2018 03:03:43] WARN: failed to generate configuration

[1] 4bcacfd2c05f45171c011d4af7731c6f848819a4

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>