runtime: Set b:undo_ftplugin where missing (#12943)

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/pbtxt.vim

@@ -2,20 +2,16 @@
 " Language:             Protobuf Text Format
 " Maintainer:           Lakshay Garg <lakshayg@outlook.in>
 " Last Change:          2020 Nov 17
+"                       2023 Aug 28 by Vim Project (undo_ftplugin)
 " Homepage:             https://github.com/lakshayg/vim-pbtxt
 
 if exists("b:did_ftplugin")
   finish
 endif
-
 let b:did_ftplugin = 1
 
-let s:cpo_save = &cpo
-set cpo&vim
-
 setlocal commentstring=#\ %s
 
-let &cpo = s:cpo_save
-unlet s:cpo_save
+let b:undo_ftplugin = "setlocal commentstring<"
 
 " vim: nowrap sw=2 sts=2 ts=8 noet