selftests: fib-onlink: Remove "wrong nexthop device" IPv6 tests

selftests: fib-onlink: Remove "wrong nexthop device" IPv6 tests

The command in the test fails as expected because IPv6 forbids a nexthop
device mismatch:

 # ./fib-onlink-tests.sh -v
 [...]
 COMMAND: ip -6 ro add table 1101 2001:db8:102::103/128 via 2001:db8:701::64 dev veth5 onlink
 Error: Nexthop has invalid gateway or device mismatch.

 TEST: Gateway resolves to wrong nexthop device - VRF      [ OK ]
 [...]

Where:

 # ip route get 2001:db8:701::64 vrf lisa
 2001:db8:701::64 dev veth7 table 1101 proto kernel src 2001:db8:701::1 metric 256 pref medium

This is in contrast to IPv4 where a nexthop device mismatch is allowed
when "onlink" is specified:

 # ip route get 169.254.7.2 vrf lisa
 169.254.7.2 dev veth7 table 1101 src 169.254.7.1 uid 0
 # ip ro add table 1101 169.254.102.103/32 via 169.254.7.2 dev veth5 onlink
 # echo $?
 0

Remove these tests in preparation for aligning IPv6 with IPv4 and
allowing nexthop device mismatch when "onlink" is specified.

A subsequent patch will add tests that verify that both address families
allow a nexthop device mismatch with "onlink".

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260111120813.159799-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>