Remove `@TagNumber(...)` annotations?

I'm unclear what the `@$pb.TagNumber(104)` annotations are used for. Thay add quite a bit of noise to the generated code - each message field is tagged w/ four `TagNumber` annotations.

```dart
/// Used as a ...
@$pb.TagNumber(103)
$core.String get apiShortName => $_getSZ(2);
@$pb.TagNumber(103)
set apiShortName($core.String v) {
$_setString(2, v);
}

@$pb.TagNumber(103)
$core.bool hasApiShortName() => $_has(2);
@$pb.TagNumber(103)
void clearApiShortName() => $_clearField(103);
```

Do we know of any use cases / users of these annotations? (or, if they are being used, why we're tagging all for items for the field instead of just something like the getter)

The docs for the annotation say; `'Annotation for marking accessors that belong together.\n\nAllows tooling to associate related accessors'`.

cc @osa1 (and also @sigurdm, who may also have context here)