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/modconf.vim b/runtime/ftplugin/modconf.vim
index 09ad54b..d5eda5a 100644
--- a/runtime/ftplugin/modconf.vim
+++ b/runtime/ftplugin/modconf.vim
@@ -28,7 +28,7 @@
   if exists(':Sman') == 2
     setlocal iskeyword+=-
     setlocal keywordprg=:Sman
-    let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword<'
+    let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword< | sil! delc -buffer Sman'
   endif
 endif