[libc++] Remove atomic_wait benchmarks

The atomic_wait benchmarks are great, but in fact they are a bit
too great. Indeed, they are so thorough that they overload the system
they're running on, meaning they can't be run on our CI infrastructure.
The NxN benchmarks even crash my system on macOS.

This patch removes the benchmarks since we're trying to move towards
more continuous benchmarking, which requires making a tradeoff between
exhaustiveness and ability to run the benchmarks on a regular basis.

Instead, it would be better to rework the benchmarks to make them
more lightweight, but I think it is reasonable to do that at a later
time in order to unblock continuous performance monitoring ASAP.

Author: ldionne