Remove SkLinearGradient::kForce4fContext_PrivateFlag

... and related tests/gms.

We now exercise the 4f impl with raster-pipeline/burst, no need for a
special test flag.

Change-Id: If67684d2d8840b3c413db9eeebb051f59cbc5a34
Reviewed-on: https://skia-review.googlesource.com/19025
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed