Delete SkImageDecoder

Delete SkImageDecoder

This image decoding implementation has been replaced
by SkCodec in Android.

Additionally, we have replaced uses of SkImageDecoder
in Skia and Google3 with uses of SkCodec.

Now we can delete SkImageDecoder :).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1820503002
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Committed: https://skia.googlesource.com/skia/+/f799706656f2581c5bf5510d94df3fa17cce1607

Committed: https://skia.googlesource.com/skia/+/5b6e73e0c8282c4d85accbfbcecc6dee84f8a1eb

Review URL: https://codereview.chromium.org/1820503002