shared: fix binding-modifier none

Fix binding-modifier "none" which was defaulting to Super.

Commit [1] introduced the original support for "none" but the logic was
subsequently lost in f7ba35f5.

[1] 553d1248 ("desktop-shell: Allow binding-modifier weston.ini option to be none")

Fixes: #976

Fixes: f7ba35f5 ("kiosk-shell: Enable debug keybindings")
Signed-off-by: Ian Ray <ian.ray@gehealthcare.com>