It is fundamentally broken and cannot work correctly with stdx::condition_variable due to the later's support for ParkingLot notifications. I also can't think of a reasonable use case in our codebase. Luckily nothing is using it so we can just delete it and remove the temptation.

We should also delete our custom implementation of stdx::this_thread::sleep_for and sleep_until since they are just worse versions of the libstdc++ implementation.