Convert some gms to use spanified Gradients API

- this removes gradients4f and gradients4f_nodither, and they are now
  complete dups of gradients and gradients_nodither

Change-Id: I799603e60159550c8e9802296264cb50e6192b32
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1124456
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>