Remove SkNx entirely

Also cleans up the scattered references remaining in the code base
(including in files I thought I got already...).

Change-Id: I7004354b1e9cea9f9d9f45b791d8ab9ce557ba01
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/542647
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
38 files changed