Revert toPictureSync

[jonahwilliams]

flutter/flutter#108471

Reverts toPictureSync and related code

[flutter-dashboard]

This pull request was opened from and to a release candidate branch. This should only be done as part of the official Flutter release process. If you are attempting to make a regular contribution to the Flutter project, please close this PR and follow the instructions at Tree Hygiene for detailed instructions on contributing to Flutter.

Reviewers: Use caution before merging pull requests to release branches. Ensure the proper procedure has been followed.

[CaseyHillers]

FYI

org-dartlang-sdk:///flutter/lib/ui/painting.dart:4838:13: Error: The getter 'PictureRasterizationException' isn't defined for the class 'Canvas'.
 - 'Canvas' is from 'dart:ui'.
Try correcting the name to the name of an existing getter, or defining a getter or field named 'PictureRasterizationException'.
      throw PictureRasterizationException._(error, stack: image._debugStack);

[dnfield]

LGTM, thanks for doing this

[CaseyHillers]

\cc @godofredoc whose's conducting next weeks release