Remove support for the short style

[munificent]

The formatter is in a transition period from the older "short" formatting style to the new "tall" style. It currently supports both styles and uses the language version of the file to determine which style you get. Eventually, we want to remove support for the old style completely so we no longer need to maintain that code. This issue tracks that.

Here are the tasks to remove the old style:

• Decide what version of the Dart SDK will no longer support the short style. Coordinate with leads and PMs to decide when we want to remove support.
• Announce the intent to stop supporting the older style. Send a notification on all the various channels letting users know that the upcoming Dart SDK release will change the formatting of their older language versioned files. Encourage users to preemptively configure their project to use the new style on all of their files to minimize disruption.
• Remove support for short style. Delete the old AST visitor, Chunk internal representation, line splitter, and short style tests. Rejoice at the simpler, cleaner formatter codebase.
• Roll it into the Dart SDK.