Remove DESKTOP key from apps_startstop test suite
    
    This should not be set here; it is set in the 'product' (i.e.
    the flavor), so it's set to 'gnome' for Workstation live flavor,
    'kde' for KDE live flavor. Setting here overrides that and makes
    it always 'gnome', we don't want that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>