remove unused mutex from pixelref

Bug: skia:6481
Change-Id: I4f2696b947c735157a38bc0810f3cebbb18c9d77
Reviewed-on: https://skia-review.googlesource.com/13581
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed