remove SK_SUPPORT_LEGACY_WRITEPIXELSCONFIG -- dead code

BUG=skia:
R=halcanary@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/201973003

git-svn-id: http://skia.googlecode.com/svn/trunk@13834 2bbb7eff-a529-9590-31e7-b0007b416f81
11 files changed