Allow the shell to run without the screenshield

The screenshield requires gdm 3.5, which can be problematic in
jhbuild configurations, or distributions that don't use GDM as the display
manager. Allow transparent fallback to gnome-screensaver in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=683060

src/main.c

@@ -34,7 +34,6 @@ extern GType gnome_shell_plugin_get_type (void);
 
 #define SHELL_DBUS_SERVICE "org.gnome.Shell"
 #define MAGNIFIER_DBUS_SERVICE "org.gnome.Magnifier"
-#define SCREENSAVER_DBUS_SERVICE "org.gnome.ScreenSaver"
 
 #define OVERRIDES_SCHEMA "org.gnome.shell.overrides"
 
@@ -153,8 +152,6 @@ shell_dbus_init (gboolean replace)
                             MAGNIFIER_DBUS_SERVICE, FALSE,
   /* ...and the org.freedesktop.Notifications service. */
                             "org.freedesktop.Notifications", FALSE,
-  /* ...and the org.gnome.ScreenSaver service. */
-                            SCREENSAVER_DBUS_SERVICE, FALSE,
                             NULL);
   /* ...and the on-screen keyboard service */
   shell_dbus_acquire_name (bus,