selftests: mlxsw: Remove deprecated test cases

After adding the previous patches, the constraint that all the router
interface MAC addresses have the same prefix is no longer relevant.

Remove the test cases that validated that this constraint is honored.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>