Remove virtual bot gpu-threaded-compositing

After adding the virtual bot gpu-threaded-compositing, the related flags
"--enable-accelerated-2d-canvas" and "--enable-threaded-compositing" are
never added properly in VirtualTestSuites. So this virtual bot is not needed
for any tests to run or test any special configuration.

Change-Id: Ifda247630d94397d0abe40cd59f1ea1272ef7361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898108
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712286}

third_party/blink/web_tests/virtual/gpu-threaded-compositing/fast/canvas/gpu-threaded-compositing/README.txt

-Tests that ensure that "--enable-accelerated-2d-canvas", "--enable-threaded-compositing" are enabled.
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