SInce this is a subset of `always_specify_types` I think we can safely remove it.

In practice, if you violate you get doubly nagged:
```
[lint] Type annotate public APIs. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
[lint] Declare method return types. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
```

One warning is probably enough to get the message across? ;)