SkEncodeImage: no more link-time registration

Also, no more SkImageEncoder class.

SK_SUPPORT_LEGACY_IMAGE_ENCODER_CLASS now only guards some
old API shims.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5006

Change-Id: I3797f584f3e8e12ade10d31e8733163453725f40
Reviewed-on: https://skia-review.googlesource.com/5006
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Reed <reed@google.com>
35 files changed