selftests: forwarding: gre_multipath: Drop IPv6 tests

Support for device-only IPv6 multipath next hops was dropped in
commit 33bd5ac5 ("net/ipv6: Revert attempt to simplify route replace
and append") and as of commit b5d2d75e ("net/ipv6: Do not allow
device only routes via the multipath API"), attempts to add a next hop
like that yield an explicit diagnostic.

Correspondingly, drop the IPv6 parts of GRE multipath test that are
supposed to test that code.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>