Remove more SkColorSpaceXform (and friends)

- gammaencodedpremul GM was just demonstrating something that we
  understand well (and have much better testing for).
- readpixels GM was filled with workarounds for things that are no
  longer true (unpremul images, clamped F16).
- Other uses can be switched to SkConvertPixels trivially.
- Remove SkColorSpaceXformPriv and SkColorLookUpTable, all unused.
- Remove SkColorSpaceXform_skcms.cpp, no longer referenced by clients.

Bug: skia:
Change-Id: I7298bb53aa61b49ad1398ebc504d35c119fd5cf4
Reviewed-on: https://skia-review.googlesource.com/157153
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
17 files changed