p13n: fix thumbs up/down on hover highlight

remove the background colour when the user hovers over thumbs up or
down.

Screenshot: https://screenshot.googleplex.com/4xfKCnudjrbPfyc

Bug: b:324247078
Test: manually verify the UI.
Change-Id: I9b11f6baeda2270f08bb8edfb35b956bb68fe6ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5410734


Commit-Queue: Nikki Moteva <moteva@google.com>
Reviewed-by: Jason Thai <jasontt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280924}