Delete GrBackendSurface's GrPixelConfig

This member variable is no longer used

TBR=bsalomon@google.com
Bug: skia:6718
Change-Id: I84680c8c3bc36eefa603f5be7f3c15b496a79948
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229478
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
33 files changed