Prune some license files that dep added

Dep apparently decided to add some license files for packages that we
aren't using, which is particularly irksome because the license is
GPL.  Remove those packages so that there's no confusion.

Issue kubernetes/sig-release#223

Makefile

@@ -486,6 +486,7 @@ dep-ensure: dep-prereqs
 	rm -rf vendor/k8s.io/code-generator/cmd/set-gen/
 	rm -rf vendor/k8s.io/code-generator/cmd/go-to-protobuf/
 	rm -rf vendor/k8s.io/code-generator/cmd/import-boss/
+	rm -rf vendor/github.com/docker/docker/contrib/
 	make bazel-gazelle