Fix mesh library remove selected item menu option

Fixes #45969.

(cherry picked from commits 189acbee60f3d3149577db5446d9115352f60216
and aa9d2149e74233b834d168a725c8b119cc72619c)

editor/plugins/mesh_library_editor_plugin.cpp

@@ -229,8 +229,8 @@ void MeshLibraryEditor::_menu_cbk(int p_option) {
 		} break;
 		case MENU_OPTION_REMOVE_ITEM: {
 			String p = editor->get_inspector()->get_selected_path();
-			if (p.begins_with("/MeshLibrary/item") && p.get_slice_count("/") >= 3) {
-				to_erase = p.get_slice("/", 3).to_int();
+			if (p.begins_with("item") && p.get_slice_count("/") >= 2) {
+				to_erase = p.get_slice("/", 1).to_int();
 				cd_remove->set_text(vformat(TTR("Remove item %d?"), to_erase));
 				cd_remove->popup_centered(Size2(300, 60));
 			}