ShellAppSystem: Only search gnomecc.menu for preferences

settings.menu was removed in gnome-menus commit
b68bcd27f44ce2c494f6e3cd9695890b9c02af04; gnomecc.menu is the intended
replacement.

(On Red Hat Linux derived systems, settings.menu continues to exist)

https://bugzilla.gnome.org/show_bug.cgi?id=645063