lib: fix removal of yang non-presence containers

lib: fix removal of yang non-presence containers

Non-presence containers don't have "destroy" callbacks. So, once
a np-container is deleted, we need to call the "destroy" callbacks
of its child nodes instead.

This commit doesn't fix any real problem as of now since all
np-containers from the FRR YANG modules contain or one more mandatory
child nodes, so they can't be deleted (libyang will add missing
np-containers when validating data). Nevertheless, upcoming YANG
modules should benefit from this change.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>