remove dead mipmap code from SkBitmap

BUG=skia:
R=fmalita@google.com, fmalita@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/271693002

git-svn-id: http://skia.googlecode.com/svn/trunk@14611 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed