Remove the Emacs mode for LCB editing.

I've written a [new and greatly-improved
`lcb-mode`](https://github.com/peter-b/lcb-mode) for Emacs from
scratch.  It will be available as a package to install from within
Emacs using MELPA.

This removes the existing `contrib/emacs/livecode-mode.el` from Emacs
because I don't plan to maintain it any more and no-one else appears
to use it.

Author: peter-b