gdebugcontroller: Drop dup_default() method as it’s broken

If `GDebugControllerDBus` remains as the only, or default,
implementation of `GDebugController`, `dup_default()` cannot work.
`GDebugControllerDBus` requires a `GDBusConnection` at construction
time, which the `GIOModule` construction code can’t provide it.

Either we use a default D-Bus connection (but which one? and how would
it be changed by the user later if it was the wrong one?), or delegate
singleton handling of the `GDebugController` to the user.

The latter approach seems more flexible.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1190

docs/reference/gio/gio-sections-common.txt

@@ -4215,7 +4215,6 @@ GDBusObjectManagerServerPrivate
 GDebugController
 GDebugControllerInterface
 G_DEBUG_CONTROLLER_EXTENSION_POINT_NAME
-g_debug_controller_dup_default
 g_debug_controller_get_debug_enabled
 g_debug_controller_set_debug_enabled
 <SUBSECTION Standard>

gio/gdebugcontroller.c

@@ -50,23 +50,6 @@
 G_DEFINE_INTERFACE_WITH_CODE (GDebugController, g_debug_controller, G_TYPE_OBJECT,
                               g_type_interface_add_prerequisite (g_define_type_id, G_TYPE_INITABLE))
 
-/**
- * g_debug_controller_dup_default:
- *
- * Gets a reference to the default #GDebugController for the system.
- *
- * Returns: (not nullable) (transfer full): a new reference to the default #GDebugController
- *
- * Since: 2.72
- */
-GDebugController *
-g_debug_controller_dup_default (void)
-{
-  return g_object_ref (_g_io_module_get_default (G_DEBUG_CONTROLLER_EXTENSION_POINT_NAME,
-                                                 "GIO_USE_DEBUG_CONTROLLER",
-                                                 NULL));
-}
-
 static void
 g_debug_controller_default_init (GDebugControllerInterface *iface)
 {

gio/gdebugcontroller.h

@@ -68,9 +68,6 @@ struct _GDebugControllerInterface {
   GTypeInterface g_iface;
 };
 
-GLIB_AVAILABLE_IN_2_72
-GDebugController      *g_debug_controller_dup_default           (void);
-
 GLIB_AVAILABLE_IN_2_72
 gboolean               g_debug_controller_get_debug_enabled     (GDebugController *self);
 GLIB_AVAILABLE_IN_2_72