[flutter_tools] delete flutter format command (#129360)

On the current stable branch (3.10.x), `flutter format` no longer works, and gives an error. This change just deletes the code.

Fixes https://github.com/flutter/flutter/issues/115809
https://dart.googlesource.com/external/github.com/flutter/flutter/+/6f3b07811b5689bffa47f39d953e1ba544e1290b