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

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

runtime/ftplugin/usd.vim

@@ -2,17 +2,13 @@
 " Language:     Pixar Animation's Universal Scene Description format
 " Maintainer:   Colin Kennedy <colinvfx@gmail.com>
 " Last Change:  2023 May 9
+"               2023 Aug 28 by Vim Project (undo_ftplugin)
 
 if exists("b:did_ftplugin")
   finish
 endif
-
-let s:cpo_save = &cpo
-set cpo&vim
-
 let b:did_ftplugin = 1
 
 setlocal commentstring=#\ %s
 
-let &cpo = s:cpo_save
-unlet s:cpo_save
+let b:undo_ftplugin = "setlocal commentstring<"