cleanup: remove DEVSTACK_CINDER_SECURE_DELETE

CINDER_SECURE_DELETE is deprecated in devstack as of liberty.
See:
https://review.openstack.org/#/c/421253/
https://review.openstack.org/#/c/465513/

Change-Id: I4f4c7a63b99620d63cb7866bf67522081609c12e

cinder/tests/tempest/README.rst

@@ -40,7 +40,7 @@ work. Update backend information to fit your environment.
                      n-api,n-cond,n-cpu,n-crt,n-net,n-sch,rabbit,tempest
     CINDER_ENABLED_BACKENDS=lvmdriver-1
     CINDER_DEFAULT_VOLUME_TYPE=lvmdriver-1
-    CINDER_SECURE_DELETE=False
+    CINDER_VOLUME_CLEAR=none
     TEMPEST_ENABLED_BACKENDS=lvmdriver-1
     TEMPEST_VOLUME_DRIVER=lvmdriver-1
     TEMPEST_VOLUME_VENDOR="Open Source"