Make sure exiting todo-mode buffer buries it (bug#27121)

This failed due to commit ea3ae33b from 2013-05-16, which prevented
quitting todo-mode buffer after visiting todo-archive buffer from
making the archive buffer current again.  Avoid this now by simply
killing the archive buffer, since there's no need to keep it a live
buffer.  Consequently, quitting a todo-mode buffer can now use
bury-buffer without an argument, which ensures that is will not
becomes current on quitting the buffer that replaced it in the window.

* lisp/calendar/todo-mode.el (todo-quit): Kill todo-archive-mode
buffer instead of burying it.  This now allows exiting the
todo-mode buffer by bury-buffer without an argument, so do that.

lisp/calendar/todo-mode.el

@@ -820,14 +820,10 @@ buries it and restores state as needed."
 	       (message "There is no todo file for this archive")))
 	   ;; When todo-check-file runs in todo-show, it kills the
 	   ;; buffer if the archive file was deleted externally.
-	   (when (buffer-live-p buf) (bury-buffer buf)))
+	   (when (buffer-live-p buf) (kill-buffer buf)))
 	  ((eq major-mode 'todo-mode)
 	   (todo-save)
-	   ;; If we just quit archive mode, just burying the buffer
-	   ;; in todo-mode would return to archive.
-	   (set-window-buffer (selected-window)
-			      (set-buffer (other-buffer)))
-	   (bury-buffer buf)))))
+	   (bury-buffer)))))
 
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