lib: fix handling of deleted nodes in the sysrepo plugin

lib: fix handling of deleted nodes in the sysrepo plugin

Make the sysrepo plugin ignore the deletion of configuration
nodes that don't exist anymore instead of logging an error and
rejecting the changes. This is necessary because Sysrepo delivers
delete notifications for all nodes of a deleted data tree instead
of delivering a single delete notification of the top-level subtree
node (which would suffice for the northbound layer).

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