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

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.