fix(swingset): comms: deleteRemoteMapping might free the object

A "downstream retire" happens when the upstream/exported Remotable is
deleted (after all importers have dropped it). This triggers retirement from
the exporting side, rather than the importing side. The inbound
`dispatch.retireImport` or remote `retireImport` message causes the exporting
c-list entry to be removed. This doesn't change any refcounts (only importing
c-list entries influence the refcount), but it should still trigger a
`maybeFree` check. `processMaybeFree()` will notice the missing exporting
c-list entry, and will schedule notifications to all remaining importers.

packages/SwingSet/src/vats/comms/remote.js

@@ -142,6 +142,10 @@ export function makeRemote(state, store, remoteID) {
       if (isImport) {
         state.removeImporter(lref, remoteID);
         deleteLastSent(lref);
+      } else {
+        // deleting the upstream/export-side mapping should trigger
+        // processMaybeFree
+        state.lrefMightBeFree(lref);
       }
     }
   }