selftests: mirror_gre_lag_lacp: Drop unnecessary code

The selftest does not use functions from mirror_gre_lib, ditch the import.

It does not use arping either, so drop the require_command as well.

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