Delete clang-completion-mode.el.

It has been unmaintained for a while (last change was more than four
years ago), and it appears not widely used.
By now there are multiple well-maintained alternatives (emacs-ycmd,
atuo-complete-clang), and if users try to make this work they'll likely
have a bad user experience.

Reasoning and problems pointed out by Philipp Stephani.

llvm-svn: 283864