Delete mixed samples

Mixed samples is no longer relevant for Ganesh. DMSAA and the new
Ganesh architecture both rely on full MSAA, and any platform where
mixed samples is supported will ultimately not use the old
architecture.

Change-Id: I5acc745010e090ef26310d92ec6240be2cd494cf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/399837
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
111 files changed