Clean up copy image experiment code

Bug:1043329

Change-Id: I82c109cc0ded499cbcb77e74026152844ac21136
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387120


Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Gang Wu <gangwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805623}