Remove kContext node type introduced in r13936

It has been decided not to pollute node types with extra values.

Review URL: https://codereview.chromium.org//12833005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13959 ce2b1a6d-e550-0410-aec6-3dcde31c8c00