arc/gio: Remove redundant focus on the plus button

This CL removes the redundant focus on the plus button as required,
removing the focus ring as well.

Bug: b/331643468
Test: manual test
Change-Id: Ie0c5b650b22b033758826adab6fe737e937bc0bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404135


Commit-Queue: Cici Ruan <cuicuiruan@google.com>
Reviewed-by: David Jacobo <djacobo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280363}