Might be of interest, even it´s related to sensors instead of groups. Seems like there´s no clean deletion/cleanup done in deCONZ itself…
After removing 2 Ikea lights from Deconz they were still available in the Integration. Calling the service deconz.remove_orphaned_entries directly from the services tab removed them finally.
Thanks.
Reading this thread from bottom to top could have saved me a lot of time 
“deconz.remove_orphaned_entries”
Fantastic
That’s what the SOLVED feature was made for. Seems it can only be applied by thread author and mods.