Fix removal of storage directory on shared file storage

This patch makes _RemoveDisks symmetric to _CreateDisks with respect
to file-based storage: _CreateDisks uses "in constants.DTS_FILEBASED",
whereas _RemoveDisks was not update and only uses "==
constants.DT_FILE". This results in stale directories left on the
filesystem.

Fixes issue 262.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>