Remove SkColorSpace::Gamut enum

Android's use of this has been removed

Bug: skia:
Change-Id: I32399f7c07efa5dea868837bd0f7fcddecdbb035
Reviewed-on: https://skia-review.googlesource.com/c/186544
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>