Comply to CNI 0.5 spec by always returning nil on DEL.
Comply to CNI 0.5 spec by always returning nil on DEL.

See also:
  "Plugins should generally complete a DEL action without error even if some resources are missing."
in:
  https://github.com/containernetworking/cni/blob/v0.5.0/SPEC.md

weaveworks/weave,weaveworks/weave,weaveworks/weave,weaveworks/weave