Clean up unused SK_SUPPORT_LEGACY_IMAGE_ENCODER_CLASS code

Approval from https://review.skia.org/5006
TBR=reed@google.com

Change-Id: Ie630a3f45a830c8c2dc80a30e759d9481b8d60a5
Reviewed-on: https://skia-review.googlesource.com/5775
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
4 files changed