Remove canvas_region_id from event handlers

This code is dead now that the canvas HitRegions feature has been
removed.
crrev.com/c/3166851

Bug: 1249855
Change-Id: I4ebf3c635a5f7b2a3db9992adf8174a03de7d711
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3168812


Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923585}