Stop testing SkImageDecoder in DM/nanobench

We have already used it for comparison, and are switching forward
to using SkCodec. This also allows us to simplify the code for checking
the extensions we support for images.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1641663002

Review URL: https://codereview.chromium.org/1641663002
8 files changed