calendar file backend: support remove with CALOBJ_MOD_ONLY_THIS
calendar file backend: support remove with CALOBJ_MOD_ONLY_THIS

Support for this capability is easy:
- report removal of the detached recurrence
- report error when not found
- avoid modifying the parent (= full_object)

Distrotech/evolution-data-server,gcampax/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,gcampax/evolution-data-server,gcampax/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,gcampax/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server