Delete missing volume test case for NodeUnpublishVolume

The "should fail when the volume is missing" test for
NodeUnpublishVolume verifies that a NotFound error occurs when the
endpoint is invoked for a missing volume. However, this expectation
really only holds for volume types that are locally attached, such as
local disks or iscsi. Specifically, it would not hold for
network-attached storage where it is fine to return Ok if the mount
point does not exist.
container-storage-interface/spec#433 was filed
to improve the spec in this regard.

For now we remove the test as it seems to block more people than it
helps.

pkg/sanity/node.go

@@ -419,21 +419,6 @@ var _ = DescribeSanity("Node Service", func(sc *TestContext) {
 			Expect(ok).To(BeTrue())
 			Expect(serverError.Code()).To(Equal(codes.InvalidArgument))
 		})
-
-		It("should fail when the volume is missing", func() {
-
-			_, err := c.NodeUnpublishVolume(
-				context.Background(),
-				&csi.NodeUnpublishVolumeRequest{
-					VolumeId:   sc.Config.IDGen.GenerateUniqueValidVolumeID(),
-					TargetPath: sc.StagingPath,
-				})
-			Expect(err).To(HaveOccurred(), "failed to unpublish volume from node")
-
-			serverError, ok := status.FromError(err)
-			Expect(ok).To(BeTrue(), "error from NodeUnpublishVolume is not a gRPC error")
-			Expect(serverError.Code()).To(Equal(codes.NotFound), "unexpected error code")
-		})
 	})
 
 	Describe("NodeStageVolume", func() {