hark-graph-hook: fix muting on %remove-graph
*_watching was bunting to watching (i.e. a no-op), causing watching to
be wiped everytimne a graph was removed
++ remove-graph
|= rid=resource
=/ unwatched
- %- ~(gas in *_watching)
+ %- ~(gas in *(set [resource index:graph-store]))
%+ skim ~(tap in watching)
|= [r=resource idx=index:graph-store]
=(r rid)
