Removes old selection code and any related methods

This patch removes the deprecated accessibility selection code from Blink.
This is timely, because the new selection code has been on for 1.5 weeks and
no major regressions have been encountered.

R=aleventhal@chromium.org, dtseng@chromium.org

Bug: 639340
Change-Id: Ibdf8a7575e229407fe3418a4efd5eed96b7b8540
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620967
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662206}