chromeos: removes arc_gpu.mojom

It's not used.

BUG=964460
TEST=none
TBR=tsepez@chromium.org

Change-Id: I6fe4da72f5632cfa7b3d9d2b21a7f844945fd499
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628954
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663257}
8 files changed