Remove _LIBCPP_DEBUG.  This was my first attempt at debug mode for libc++, and is now obsoleted by _LIBCPP_DEBUG2 (which isn't finished).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189135 91177308-0d34-0410-b5e6-96231b3b80d8