Remove the onboarding WG, it was not formal working group
The onboarding working group was discussed .. I think in 2019, then kind-of-announced on the membership list. Only in May 2021 did we add a page listing it as a WG. In 2020 and 2021 the WG did not report at the AGM. This MR removes the WG again, and we can conclude that it didn't really work out.