Delete SkImage::encode(Type)

Delete SkImage::encode(Type)

This doesn't appear to be used (fingers crossed).

We're not ready to support color correct encodes (and tagging with
color spaces) for all of the formats included in SkImageEncoder::Type.

I think it's good to restrict this for now.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4535

Change-Id: I73000f3d7bff1af153786d766531a2cbc8c5b77a
Reviewed-on: https://skia-review.googlesource.com/4535
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>