Remove SK_SUPPORT_LEGACY_BILERP.

Change-Id: I38b9154cdc1e2ee064158af9d92d08d1c6297458
Reviewed-on: https://chromium-review.googlesource.com/575675
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487507}