Remove unused Debug::AfterCompileFlags flags.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/353823004

Patch from Alexey Kozyatinskiy <kozyatinskiy@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22042 ce2b1a6d-e550-0410-aec6-3dcde31c8c00