Unix sock file is not removed after the server stop

From the [doc](https://github.com/kubernetes-csi/csi-driver-host-path/blob/master/pkg/hostpath/README.md), when the server started the default `/tmp/csi.sock` file was created automatically.

But after kill the server, the sock file was not removed accordingly.
```
ll /tmp

srwxr-xr-x 1 dev  dev     0 Oct  6 17:07 csi.sock
```