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/systemd.vim b/runtime/ftplugin/systemd.vim
index c0bc630..e60a5e4 100644
--- a/runtime/ftplugin/systemd.vim
+++ b/runtime/ftplugin/systemd.vim
@@ -30,7 +30,7 @@
     if !exists('b:undo_ftplugin') || empty(b:undo_ftplugin)
       let b:undo_ftplugin = 'setlocal keywordprg< iskeyword<'
     else
-      let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword<'
+      let b:undo_ftplugin .= '| setlocal keywordprg< iskeyword< | sil! delc -buffer Sman'
     endif
   endif
 endif