Delete SkBitmapRegionCanvas

This was an approach we considered for implementing Android's
BitmapRegionDecoder.

It was useful for testing and comparison, but now is no longer worth
maintaining and testing.

The approach to subset/scaled decodes (clipped decode, then scale)
may be worth reconsidering at some point.

BUG=skia:5307
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1990543002

Review-Url: https://codereview.chromium.org/1990543002
11 files changed