remove Sk4x4f.h -- dead code

Change-Id: I03d15a563eeb7ea187c7d48deffe00357dc55177
Reviewed-on: https://skia-review.googlesource.com/c/188032
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed