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csi: Remove stale volume path

**What this PR does / why we need it**:

The CSI mounter creates the following paths during SetUp():

   * .../pods/\<podID\>/volumes/kubernetes.io~csi/\<specVolId\>/mount/
   * .../pods/\<podID\>/volumes/kubernetes.io~csi/\<specVolId\>/volume_data.json

During TearDown(), it does not remove the `.../kubernetes.io~csi/<specVolId>/`
directory, leaving behind orphan volumes: method cleanupOrphanedPodDirs()
complains with 'Orphaned pod found, but volume paths are still present
on disk'.

Fix that by removing the above directory in removeMountDir().

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```