[runtimes] Remove the ability to select the old libc++ testing format

As announced on libcxx-dev at [1], the old libc++ testing format is being
removed in favour of the new one. Follow-up commits will clean up the
code that is dead after the removal of this option.

[1]: http://lists.llvm.org/pipermail/libcxx-dev/2020-June/000885.html