selftests: forwarding: Remove IPv6 L3 multipath hash tests

selftests: forwarding: Remove IPv6 L3 multipath hash tests

The multipath tests currently test both the L3 and L4 multipath hash
policies for IPv6, but only the L4 policy for IPv4. The reason is mostly
historic: When the initial multipath test was added
(router_multipath.sh) the IPv6 L4 policy did not exist and was later
added to the test. The other multipath tests copied this pattern
although there is little value in testing both policies.

Align the IPv4 and IPv6 tests and only test the L4 policy. On my system,
this reduces the run time of router_multipath.sh by 89% because of the
repeated ping6 invocations to randomize the flow label.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240304095612.462900-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>