Minor cleanup (remove unused GrRenderTargetContext::asTexture method)

Change-Id: I719204e9870b3c849a5c60d967359a70138e6c9c
Reviewed-on: https://skia-review.googlesource.com/10753
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed