Remove dead debug_mode doc link

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290659 91177308-0d34-0410-b5e6-96231b3b80d8

docs/index.rst

diff --git a/docs/index.rst b/docs/index.rst
index c62dfe8..8f21a27 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -135,7 +135,6 @@
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
 * `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_
-* `Status of debug mode <http://libcxx.llvm.org/debug_mode.html>`_
 * `Notes by Marshall Clow`__
 
 .. __: https://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/