Remove meta_get_backend()

It should be retrieved from the context via an ownership chain.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>

src/backends/meta-backend.c

@@ -116,23 +116,8 @@ enum
 
 static guint signals[N_SIGNALS];
 
-static MetaBackend *_backend;
-
 #define HIDDEN_POINTER_TIMEOUT 300 /* ms */
 
-/**
- * meta_get_backend:
- *
- * Accessor for the singleton MetaBackend.
- *
- * Returns: (transfer none): The only #MetaBackend there is.
- */
-MetaBackend *
-meta_get_backend (void)
-{
-  return _backend;
-}
-
 struct _MetaBackendPrivate
 {
   MetaContext *context;
@@ -213,8 +198,6 @@ meta_backend_dispose (GObject *object)
   MetaBackend *backend = META_BACKEND (object);
   MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
 
-  _backend = NULL;
-
   g_clear_pointer (&priv->cursor_tracker, meta_cursor_tracker_destroy);
   g_clear_object (&priv->current_device);
   g_clear_object (&priv->color_manager);
@@ -1236,7 +1219,6 @@ initable_iface_init (GInitableIface *initable_iface)
 static void
 meta_backend_init (MetaBackend *backend)
 {
-  _backend = backend;
 }
 
 /**

src/meta/meta-backend.h

@@ -43,9 +43,6 @@ typedef enum _MetaBackendCapabilities
 META_EXPORT
 G_DECLARE_DERIVABLE_TYPE (MetaBackend, meta_backend, META, BACKEND, GObject)
 
-META_EXPORT
-MetaBackend * meta_get_backend (void);
-
 META_EXPORT
 void meta_backend_set_keymap (MetaBackend *backend,
                               const char  *layouts,