Remove remove_old_gn_binaries DEPS entry.

Marked for removal at the end of last month.

R=kjellander@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/18049004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6826 4adac7df-926f-26a2-2b94-8c16560cd09d