Remove GrContext variant of SkImageGenerator::isValid

This method isn't used by anyone, and I needed to land SOMETHING
in the middle of banging my head against CL 310656 :-)

Bug: skia:104662
Change-Id: Iaf529ac7551b22213891eb4be78d7904938aa989
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311816
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed