Cleanup OptimizedRealtimeThreadingMac from fieldtrial_testing_config.json.

R=olka@chromium.org

Bug: 1289680
Change-Id: Ic705587f2246da8497245142c5f5c7a1f92a2068
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3412653


Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962538}