Remove extraneous delete statement

The close handler will be called on cancel anyway, so we don't need
to delete in the click handler.

(imported from commit 0fcf4b0d1408312a0889f2b69e01207c9c3835fa)

zephyr/static/js/notifications.js

@@ -149,7 +149,6 @@ function process_desktop_notification(message) {
     notification_object = notice_memory[key].obj;
     notification_object.onclick = function () {
         notification_object.cancel();
-        delete notice_memory[key];
         window.focus();
     };
     notification_object.onclose = function () {