Prune old flaky WebGPUExpectations

Change-Id: I71ee8eee09ca35cc2c44c75d1a01a06c29a1b853
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3072000
Auto-Submit: Austin Eng <enga@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#908593}