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Link class used to have two focus styles: focus ring and underline. Now
we always use underline focus style. So remove all the code related to
supporting different focus styles.

BUG=none
TEST=nothing should change.

Change-Id: I80b826ff246e0fbfd18c73f79b17f7e34f43a740
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138781
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757202}