selftests: forwarding: ipip_lib: Do not import lib.sh

This library is always sourced in the context where lib.sh has already been
sourced as well. Therefore drop the explicit sourcing and expect the client
to already have done it. This will simplify moving some of the clients to a
different directory.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/a4da5e9cd42a34cbace917a048ca71081719d6ac.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>