Remove non-F32 specializations of Sk4fGradient

We're only using Sk4fGradients for raster pipeline burst mode => dest is
always F32.

Change-Id: If8f0ce257fc9ef36da33602ffd61617628733cfc
Reviewed-on: https://skia-review.googlesource.com/20280
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed