@HeMaN - if you're still looking to clean up the old guest-tools.iso, give this a try:
xe vdi-list sr-uuid=<uuid of XCP-ng Tools SR>
This will give you a list of VIDs. Find the one you want to remove and then execute:
xe vdi-forget uuid=<uuid of VID>
Rinse and repeat till all the old guest tools are removed and you won't see them in XOA.