I did that months ago. It doesn't work. Fedora 23 is still in the XFCE Applications menu.
http://i.imgur.com/wkrB0BT.png
I think its alt f3 to get the editing mode for start menu.
Did you try including "*" at the end of fedora23? It's not there in your pic.
rm ~/.local/share/applications/fedora23*
This works to hide it but hidden is still there in a sense. Seems a little goofy that there's no way to do this, no?
I understand Qubes has its parts separate intentionally but I think there's a problem in some places, like this one; where you aren't able to do something as simple as remove a menu entry.
Just my 2c if there really is no way to do this properly.