(remove SK_SUPPORT_LEGACY_RADIAL_GRADIENT)

Change-Id: I0b6c2b97d84f4566bd661a43923b30d7359aa321
Reviewed-on: https://chromium-review.googlesource.com/581725
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489059}