Remove GPU read-back logic in SkSpecialImage

All image filters are now implemented entirely on the GPU, so we never
need to read back the contents of a texture-backed special image.

Bug: skia:10202
Change-Id: I9e814d4bccde1e638f7bfc27b140e010ddcbcdb9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298138
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed