remove unused SkNx::invert()

The instructions used to implement it don't deliever
the same results across processors, even on the same
architecture.

Bug: skia:11861
Change-Id: Ifb87fc04a4ec866f84d7c2f2472c7f4b65661d28
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396721
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed