Remove Subset*Benches

These were originally used to compare to the old implementation of
subset decoding in SkImageDecoder. The old implementation has been
removed, and they do not provide useful information that is not
covered by the BitmapRegionDecoderBenches.

This will greatly speed up some of our infra bots, which spend a lot of
time decoding interlaced PNGs repeatedly (thanks to
SubsetTranslateBench).

BUG=skia:4715
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1531833002

Review URL: https://codereview.chromium.org/1531833002
8 files changed