Remove SkBase64 from public API

Client CLs:
- https://github.com/flutter/engine/pull/46543

Change-Id: Ifd06cfa574be69bdfcbbd02e7f8e224a47493dcb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/764176
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
19 files changed