Fix #129266: GPv3: Layer Groups hide masks by default

Previously, we initialized the layers and groups to hide masks.
For layers this makes sense, because they don't have any masks
by default, but for groups it makes more sense to show masks
by default for all the layers inside.

Pull Request: https://projects.blender.org/blender/blender/pulls/129528

source/blender/blenkernel/intern/grease_pencil.cc

@@ -3470,8 +3470,6 @@ blender::bke::greasepencil::LayerGroup &GreasePencil::add_layer_group(
                                                    name.c_str();
   bke::greasepencil::LayerGroup *new_group = MEM_new<bke::greasepencil::LayerGroup>(__func__,
                                                                                     unique_name);
-  /* Hide masks by default. */
-  new_group->base.flag |= GP_LAYER_TREE_NODE_HIDE_MASKS;
   return parent_group.add_node(new_group->as_node()).as_group();
 }