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 <[email protected]>
Link: https://lore.kernel.org/r/a4da5e9cd42a34cbace917a048ca71081719d6ac.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: pmachata

tools/testing/selftests/net/forwarding/ipip_lib.sh

@@ -141,7 +141,6 @@
 # |               $h2 +       |
 # |     192.0.2.18/28         |
 # +---------------------------+
-source lib.sh
 
 h1_create()
 {