popovers: Move scroll handler to popover_menus module.

Part of cleaning out the original legacy popovers system.

web/src/click_handlers.js

@@ -669,7 +669,6 @@ export function initialize() {
         });
     }
 
-    popovers.register_click_handlers();
     user_profile.register_click_handlers();
     stream_popover.register_click_handlers();