Remove DEPS reference to third_party/clang_format

Clang format has moved into Chromium's src/buildtools
and the last traces from third_party/clang_format were
removed in http://crrev.com/285030.

This removes it from the WebRTC checkouts as well (it is
now an tree of empty directories).

Our DEPS entry for removing the old binaries from pre-move
into src/buildtools was removed in
https://code.google.com/p/webrtc/source/detail?r=6788

BUG=
R=henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21259004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6936 4adac7df-926f-26a2-2b94-8c16560cd09d