Remove DesktopAndroidUnifiedCompositorLimits from fieldtrial_testing_config.json

We enabled this in https://crrev.com/c/8490785 to validate a bug fix.
Clearing out the config.

Change-Id: Idb86a1c88457d3eb5a1757cb379aafb5345e27de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8583344
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1719653}
NOKEYCHECK=True
GitOrigin-RevId: 89c4b312d264f37c0e1aaa4cd6f5f5cf3d5a309a