remove SkColorSpaceTransferFn::invert()

Looks like DM may be its only user?
That was me and just for convenience... I can fold it through.

Change-Id: I4ed6ee659df4192e3d6db7ce5b055c4c923673bd
Reviewed-on: https://skia-review.googlesource.com/c/159761
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed