[libc++] Drop the legacy debug mode symbols by default

Leave the escape hatch in place with a note, but don't include the
debug mode symbols by default since we don't support the debug mode
in the normal library anymore.

This is technically an ABI break for users who were depending on
those debug mode symbols in the dylib, however those users will
already be broken at compile-time because they must have been using
_LIBCPP_DEBUG=2, which is now an error.

Differential Revision: https://reviews.llvm.org/D127360