Remove unused code from Blink

It seems ProxyServer.cpp|.h is not used anywhere in blink. I just came across this code while
working on sth else and don't really know why we have it, etc
but asking for a review anyway.

Review URL: https://codereview.chromium.org/687683003

git-svn-id: svn://svn.chromium.org/blink/trunk@184756 bbb929c8-8fbe-4397-9dbb-9b2b20218538