unlockDialog: Don't make dialog focusable

There is little point in focusing the dialog itself, we want keynav
to navigate inside instead and navigate between focusable children.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2210

js/ui/unlockDialog.js

@@ -476,7 +476,6 @@ var UnlockDialog = GObject.registerClass({
             accessible_role: Atk.Role.WINDOW,
             style_class: 'login-dialog',
             visible: false,
-            can_focus: true,
             reactive: true,
         });