GPv3: Wrong onion skinning icon in the dopesheet

`PROP_ICONS_CONSECUTIVE` is set for `use_onion_skinning` property with the
help of `RNA_def_property_ui_icon`. In this case, `but->iconadd` will
handle the other icon for "toggle" button types. So just use
`ICON_ONIONSKIN_OFF` when drawing the onion skinning button in dopesheet.

Resolves #121232

Pull Request: https://projects.blender.org/blender/blender/pulls/121254

source/blender/editors/animation/anim_channels_defines.cc

@@ -5572,13 +5572,12 @@ static void draw_grease_pencil_layer_widgets(bAnimListElem *ale,
   if (RNA_path_resolve_property(
           &id_ptr, onion_skinning_rna_path->c_str(), &ptr, &onion_skinning_prop))
   {
-    const int icon = layer->use_onion_skinning() ? ICON_ONIONSKIN_ON : ICON_ONIONSKIN_OFF;
     uiDefAutoButR(block,
                   &ptr,
                   onion_skinning_prop,
                   array_index,
                   "",
-                  icon,
+                  ICON_ONIONSKIN_OFF,
                   offset,
                   rect->ymin,
                   ICON_WIDTH,