Clean up BlockRef

  - It's no longer needed to help the (2011?) transition to SkAutoTUnref.
  - It prevents us from making classes that go in SkAutoTUnrefs final,
    i.e. all ref-counted classes.

This had better not have been public API...
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1068443002
1 file changed