Re-enable default features in `clap`

[sffc]

Fixes https://github.com/unicode-org/icu4x/issues/4843

Alternatively we could re-enable all the features manually, but that's brittle, and I'd rather use the default features.

We should probably check all the other crates, too. In general I think we should use default features for datagen deps.

[Manishearth]

As long as it's not pulling in more deps I'm fine with it.

[sffc]

Autosubmit = true