Remove dead code (parseFlowThread) in BisonCSSParser.h

This overload of parseFlowThread doesn't seem to be used anywhere.

Review URL: https://codereview.chromium.org/139273003

git-svn-id: svn://svn.chromium.org/blink/trunk@165144 bbb929c8-8fbe-4397-9dbb-9b2b20218538