Revert of gpu_test: enable Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing (patchset #2 id:20001 of https://codereview.chromium.org/2471173002/ )

      Reason for revert:
Failing on Mac Retina GPU FYI bot:

https://build.chromium.org/p/chromium.gpu.fyi/builders/Mac%20Retina%20Release/builds/8633

Original issue's description:
> gpu_test: enable Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing
>
> https://codereview.chromium.org/2454153002 requests r4 new revision, and cloud
> is ready.
>
> Check https://console.cloud.google.com/storage/browser/chromium-gpu-archive/reference-images/?pli=1&prefix=Pixel_OffscreenCanvasUnaccelerated2DGPUCompositing
> CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
>
> Committed: https://crrev.com/a6a0a961f10a944f931d0e18dcdf951fedf44b07
> Cr-Commit-Position: refs/heads/master@{#429666}

TBR=dongseong.hwang@chromium.org,kbr@chromium.org,dongseong.hwang@intel.com
# Not skipping CQ checks because original CL landed more than 1 days ago.

Review-Url: https://codereview.chromium.org/2479923002
Cr-Commit-Position: refs/heads/master@{#430057}