Clean up some unused atomic routines.

AtomicTest was the only use of sk_atomic_add().
AtomicInc64 bench was the only use of sk_atomic_inc(int64_t*).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2183473005
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN-Trybot,Test-Ubuntu-GCC-Golo-GPU-GT610-x86_64-Release-TSAN-Trybot

Review-Url: https://codereview.chromium.org/2183473005
3 files changed