Update uninstall-volcano.sh

helm uninstall will not delete namespace.

Signed-off-by: mahmut <mahmut@uniontech.com>

Author: mahmut-Abi

benchmark/sh/uninstall-volcano.sh

@@ -14,4 +14,5 @@
 # limitations under the License.
 set -e
 
-helm uninstall volcano --namespace volcano-system
+helm uninstall volcano --namespace volcano-system
+kubectl delete namespace volcano-system