images.viewer: Don't override model activation/deactivation
images.viewer: Don't override model activation/deactivation

`image-control` implements model activation/deactivation in graft*/ungraft*, while
this should be done already in `notify` for all gadgets.

This commit removes that code.

mrjbq7/factor,tgunr/factor,mrjbq7/factor,tgunr/factor,tgunr/factor,factor/factor,factor/factor,bpollack/factor,bpollack/factor,bpollack/factor,mrjbq7/factor,mrjbq7/factor,mrjbq7/factor,factor/factor,factor/factor,tgunr/factor,factor/factor,tgunr/factor,bpollack/factor,mrjbq7/factor,bpollack/factor,bpollack/factor,tgunr/factor,bpollack/factor,factor/factor