Revert "Add ThrottleUnimportantFrameTimers to fieldtrial testing config"

This reverts commit ec2f74f668e8e490fede52cf2a9fe0222df79972.

Reason for revert: We decided not to pursue this feature, given
no impact in a stable 50% experiment.

Bug: None
Original change's description:
> Add ThrottleUnimportantFrameTimers to fieldtrial testing config
>
> Add ThrottleUnimportantFrameTimers to fieldtrial_testing_config.json
>
> Bug: None
> Change-Id: Id3feb7c0da9108a1f7fe8018c5b76f40bc6c10cc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5010307
> Commit-Queue: Zheda Chen <zheda.chen@intel.com>
> Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
> Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1221366}

Bug: None
Change-Id: I429fad41f222317a678a00fe87cb6f14b6b3823c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578978
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469479}