pkg/ns: remove namespace creation (and move to testutils)

Namespace creation had an unergonomic interface and isn't used, except
for testing code. Remove it; downstream users should really be creating
their own namespaces

plugins/sample/sample_linux_test.go

@@ -29,7 +29,7 @@ var _ = Describe("sample test", func() {
 
 	BeforeEach(func() {
 		var err error
-		targetNs, err = ns.NewNS()
+		targetNs, err = testutils.NewNS()
 		Expect(err).NotTo(HaveOccurred())
 	})