[DevTools] Clean up Virtual Instances from id map (facebook#34063)

This was a pretty glaring memory leak. 🙈

I forgot to clean up the VirtualInstances from the id map so the Server
Component instances always leaked in DEV.

Author: sebmarkbage

packages/react-devtools-shared/src/backend/fiber/renderer.js

@@ -2749,6 +2749,8 @@ export function attach(
 
     const id = instance.id;
     pendingRealUnmountedIDs.push(id);
+
+    idToDevToolsInstanceMap.delete(instance.id);
   }
 
   function getSecondaryEnvironmentName(