Remove skstd::optional entirely.

Skia now uses C++17's std::optional.

Change-Id: I387069589baaefadd25e25bcec3f4cc6ee6fd090
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/501477
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
54 files changed