Image decoding: Remove deprecated code path

Code that uses discardable memory on Android is now deprecated. Image
cache is moved to Skia and we can remove the deprecated code safely.

Deleted about 500 lines of code.

Review URL: https://codereview.chromium.org/489323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180744 bbb929c8-8fbe-4397-9dbb-9b2b20218538