runtime: cleanup :Sman command via the undo_ftplugin mechanism (#12967)

Regards to @dkearns as noticed in
2ac708b548

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

runtime/ftplugin/zsh.vim

@@ -30,7 +30,7 @@ if executable('zsh') && &shell !~# '/\%(nologin\|false\)$'
     compiler zsh
   endif
   setlocal keywordprg=:RunHelp
-  let b:undo_ftplugin .= 'keywordprg<'
+  let b:undo_ftplugin .= 'keywordprg< | sil! delc -buffer RunHelp'
 endif
 
 let b:match_words = '\<if\>:\<elif\>:\<else\>:\<fi\>'