Remove Message::GetScriptData declaration

This is a left-over after r20683

BUG=None
LOG=N
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/266983003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21136 ce2b1a6d-e550-0410-aec6-3dcde31c8c00