Remove dep prune

[imjasonh]

Whenever I run ./hack/update-codegen.sh:

Pruning is now performed automatically by dep ensure.
Set prune settings in Gopkg.toml and it it will be applied when running ensure.

This command currently still prunes as it always has, to ease the transition.
However, it will be removed in a future version of dep.

Now is the time to update your Gopkg.toml and remove `dep prune` from any scripts.

[imjasonh]

knative/eventing#60