Deferred image decoding: clean up and remove unused scaling code

There are some code doing scaling with lanczos3 filtering in the
deferred image decoding path. This code was never used and is good to
remove.

Review URL: https://codereview.chromium.org/142073003

git-svn-id: svn://svn.chromium.org/blink/trunk@165567 bbb929c8-8fbe-4397-9dbb-9b2b20218538