selftests: mlxsw: vxlan: Remove IPv6 test case

Currently, there is a test case to verify that VxLAN with IPv6 underlay
is forbidden.

Remove this test case as support for VxLAN with IPv6 underlay was added
by the previous patch.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>