selftests: mirror: Drop dual SW/HW testing

The mirroring tests are currently run in a skip_hw and optionally a skip_sw
mode. The former tests the SW datapath, the latter the HW datapath, if
available. In order to be able to test SW datapath on HW loopbacks, traps
are installed on ingress to get traffic from the HW datapath to the SW one.
This adds an unnecessary complexity when it would be much simpler to just
use a veth-based topology to test the SW datapath. Thus drop all the code
that supports this dual testing.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>