Remove (now-dead) SK_LEGACY_SWEEP_GRADIENT code

Reland of https://skia-review.googlesource.com/c/17982

TBR=

Change-Id: If29d60d60a45562715fddfaff56b1e694be13034
Reviewed-on: https://skia-review.googlesource.com/24042
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed