Remove kImageAd Usage Counter

The counter was successfully used to verify image ad usage and is no
longer needed. The component will later be applied also to iframe ads,
and we won't differentiate between the two.

Change-Id: I07f7790cee6c1f396735a915188663e816634ca0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6891991
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508397}