libgimp: fix annotations for gimp_brush_get_by_name().

Similarly to how we handled image items, all resources are handled and stored by
the plug-in infrastructure and should not be destroyed. This wrong annotation
was triggering bindings to unref resources when going out of scope, hence
crashing plug-ins.

libgimp/gimpbrush_pdb.c

@@ -78,9 +78,7 @@ gimp_brush_new (const gchar *name)
  *
  * Returns the brush with the given name.
  *
- * Returns the brush with the given name.
- *
- * Returns: (transfer full): The brush.
+ * Returns: (transfer none): The brush.
  *
  * Since: 3.0
  **/