[//viz] Remove copy_output_util dead code

This code has no callers in the codebase, and removing it clarifies
the usage of CopyOutputResult::Format::NV12_PLANES within the codebase.

Bug: 1429004
Change-Id: I57967d68f34345f71d35d7c17bf0fb98fdf96a53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4565294


Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149130}