xdg-su: Remove quotes around lxqt-sudo cmd arg

lxqt-sudo does not use quotes for its command argument

Currently:

> lxqt-sudo -s "echo WORKS!"

bash: line 1: exec: echo WORKS!: not found

With fix:

> lxqt-sudo -s echo WORKS!

WORKS!

Currently this bug prevents any and all admin apps from executing from the start menu in the standard lxqt desktop.