Remove SkRuntimeShader::makeImage.

We don't have any known users, and it no longer exposes anything that
a user can't just do directly.

Change-Id: Id653a6be3f265a2847b1670f3e6c054cf2d094a2
Bug: skia:12482
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453142
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed