Revert "Enable libstdc++ debug mode in debug builds.".

This reverts commit r24676 to see if it was the cause for the mysterious
crashes on some bots. If not, we'll reland it.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/663013002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00