Do not register SDFMaskFilter for flattening

These don't get serialized, as they are created as necessary.

Thus they shouldn't be in any bytes that a user would have
us deserialize.

Change-Id: If44af619ac3ea0596ccf1f02077b29bf327fb185
Bug: skia:14317
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/727256
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed