[ffigen] ObjC example imports `package:ffi` unnecessarily

[liamappelbe]

We have a visitor that tries to decide which imports are needed. Ideally there would be no false positive imports, but that's very hard to guarantee after #2525.

[goderbauer]

We already generate // ignore_for_file: type=lint to disable lints. As a work around and to avoid that people individually have to deal with this, should we extend this to also ignore unused imports by generating // ignore_for_file: type=lint, unused_import instead?

[goderbauer]

I am implementing that work around in #2750.