Delete clang-completion-mode.el.
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.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@283864 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang