This reverts commit 28a03248 (and removes a
stry empty line).  There is no more need for our own /app/bin/xdg-open, as
xdg-open in the freedesktop.org (and thus also GNOME) runtime (see
<https://github.com/flatpak/flatpak-xdg-utils/blob/master/src/xdg-open.c>) by
now knows to ask the Desktop portal, too.

Change-Id: Ie4141d02ba5f43c0264afcfae9edfc2d3a8cdb16