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

Regards to @dkearns as noticed in
https://github.com/vim/vim/commit/2ac708b548660b232a32c52d89bde3d8596646c0

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/zsh.vim b/runtime/ftplugin/zsh.vim
index 0ca8077..ed75d04 100644
--- a/runtime/ftplugin/zsh.vim
+++ b/runtime/ftplugin/zsh.vim
@@ -30,7 +30,7 @@
     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\>'