Remove unused mock content browser client

Change-Id: I5620ecf74ebbe403ac4ca302342db35aed016e2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5421208


Reviewed-by: Andrew Paseltiner <apaseltiner@chromium.org>
Commit-Queue: Nan Lin <linnan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281924}