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