selftests: net: forwarding: lib: Drop lldpad_app_wait_set(), _del()

The existing users of these helpers have been converted to iproute2 dcb.
Drop the helpers.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>