Remove old flags in skia.gni

I believe Flutter was the last user of these and no longer uses them.

Users who have skia_use_gpu in their GN args.gni file should change
it to skia_use_ganesh.

Change-Id: If6b941aafc5a489279afb262a2a3c485027a0ab1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1018096
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Thomas Smith <thomsmit@google.com>
1 file changed