remove sk_atomic_dec

This turns out to be mostly deleting dead debugging code.

Change-Id: I4969ea380e6125e8b557d430c6720edc0a337a79
Reviewed-on: https://skia-review.googlesource.com/c/174284
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
14 files changed