Added recursive for deletion of $OVN_RUNDIR

After ./unstack.sh trying to 'enable_plugin venus https://opendev.org/openstack/venus' gived following error:

+lib/neutron_plugins/ovn_agent:install_ovn:363  sudo ln -s /var/run/openvswitch /var/run/ovn
ln: failed to create symbolic link '/var/run/ovn/openvswitch': File exists

which led to:

+lib/neutron_plugins/ovn_agent:cleanup_ovn:801  sudo rm -f /var/run/ovn
rm: cannot remove '/var/run/ovn': Is a directory

Change-Id: I1cafdc0c71093ed7249bb9748b57d51110986686

lib/neutron_plugins/ovn_agent

@@ -798,5 +798,5 @@ function cleanup_ovn {
         _cleanup $ovs_path
     fi
 
-    sudo rm -f $OVN_RUNDIR
+    sudo rm -rf $OVN_RUNDIR
 }