remove code directly involved in lock/unlock in SkPixelRef

Bug: skia:6481
Change-Id: I3c301ad42b082e04e233aa32d77862036fe998fa
Reviewed-on: https://skia-review.googlesource.com/13463
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed