Image decoding: Remove deprecated image caching code

ImageDecodingStore was used to cache images for impl side painting on
Android. Now impl side painting is using image cache in Skia.
ImageDecodingStore is only used to cache decoders.

Deleted 1k lines of code.

Review URL: https://codereview.chromium.org/485833005

git-svn-id: svn://svn.chromium.org/blink/trunk@180746 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r180741

http://src.chromium.org/viewvc/blink?view=revision&revision=180741

BUG=404179
TBR=jbroman@chromium.org

Review URL: https://codereview.chromium.org/498583002

git-svn-id: svn://svn.chromium.org/blink/trunk@180747 bbb929c8-8fbe-4397-9dbb-9b2b20218538