[libcxx-dev] Dropping support for the old libc++ test format

[Louis Dionne via libcxx-dev]

Folks,

I'd like to drop support for the old libc++ test format. The old format doesn't work well anymore because it's difficult to maintain both formats properly. Furthermore, after resolving some issues in the past two months, I believe the new format is now in a state where it is strictly more powerful than the old one.

If there's a reason why someone is unable to migrate to the new format, please let me know and we can work on resolving those issues. Otherwise, I'll remove the code related to the old format early next week and it won't be possible to use the old format anymore by specifying `--param use_old_format=True`.

Cheers,
Louis