remove bundled elisp library tree-widget.el

This library is part of Emacs.  Bundling it in the same directory as
the sclang*.el libraries will put it on the load-path.  The bundled
version then shadows the newer version which is part of Emacs, which
can cause hard to detect problems.
Merge pull request #840 from tarsius/master

remove bundled elisp library tree-widget.el