Removed v8::AssertNoGCScope.

Everything was private, so no object could ever be constructed, which
implies that nobody uses it. Furthermore, it contained a TODO and was
overly complicated, an #ifdef-less simple pimpl idiom would have been
enough.

LOG=y
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/128113002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18493 ce2b1a6d-e550-0410-aec6-3dcde31c8c00