selftests: forwarding: Ditch skip_on_veth()

Since the selftests that are not supposed to run on veth pairs are now in
their own dedicated directory, the skip_on_veth logic can go away. Drop it
from the selftests, and from lib.sh.

Cc: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/63b470e10d65270571ee7de709b31672ce314872.1711464583.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>