Do not include xapi-clusterd.service in toolstack.target (#6096)

This service is not enabled and started by default, but used on-demand
whenever it is needed for clustering. The current Wants= option in the
toolstack.target is causing xapi-clusterd.service to be started by
xe-toolstack-restart even if it is not enabled.

The fix is to replace Wants= in toolstack.target with WantedBy= in
xapi-clusterd.service, as the latter only installs the dependency when
enabling the service.

scripts/toolstack.target

@@ -20,7 +20,6 @@ Wants=xcp-networkd.service
 Wants=xenopsd-xc.service
 Wants=squeezed.service
 Wants=xapi-storage-script.service
-Wants=xapi-clusterd.service
 Wants=varstored-guard.service
 
 [Install]