remove sk_atomic_load and sk_atomic_store

Change-Id: I6eb4469df0315283f96631ff6023170fa80aaa8a
Reviewed-on: https://skia-review.googlesource.com/c/174281
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed