Remove debug only clearStencil virtual on GrGpu.

Bug: skia:
Change-Id: Ic68cd97675f554b60e84ec36d8068ce539cce6aa
Reviewed-on: https://skia-review.googlesource.com/c/159945
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed