Remove do-nothing MPD benchmarking from nanobench

Change-Id: Ife1f5646c0e97b666954518f8c32bb7bae926c6d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313677
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed