[Cast Convergence] Remove unused GN include. This file is being removed from Chromium. It is not needed in this file and can be removed, which will avoid breakage when that happens. Bug: angleproject:8413 Change-Id: Iccc4b45393bcbc268add434b16172e90d2b9d13b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5027723 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>