Replace SK_UNUSED with C++17 [[maybe_unused]].

We no longer need a custom macro for [[maybe_unused]], since it was
added as a standard attribute in C++17.

Change-Id: Ia11ef83f218aff28c1bd3a2060ca917a86fc165e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/724397
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed