Remove --fix

[munificent]

The --fix support in dart_style is older than the dart fix command. That command can now apply all of the fixes that dart format --fix can and many many more. It's redundant and confusing to have two ways to apply automated code clean-ups. And the implementations of the fixes in dart_style is kind of hacky and brittle. I believe there are cases where the resulting fixed code doesn't end up with the same formatting you'd get if you applied the fix separately.

Dart 3 gives us a chance to clean up our CLI user experience, so it's a great time to remove --fix from dart format and delete all of that crufty code. We can leave the options in there as hidden. If you pass any, instead of applying the fix, it will print a diagnostic telling you how to run dart fix.