Stop generating the TraitVariantBlanketType's blanket impl?

[spastorino]

Right now the crates generates ...

impl<TraitVariantBlanketType: SendNext> Next for TraitVariantBlanketType {
    type Item = <Self as SendNext>::Item;
    async fn next(&mut self) -> Option<Self::Item> {
        <Self as SendNext>::next(self).await
    }
}

And this is incompatible with https://github.com/spastorino/dynosaur once this spastorino/dynosaur#77 lands. We should discuss if we should stop doing that here or what.