Remove the `- soft-warnings` option from the test `traits.rs`

[sonmarcho]

Once https://github.com/AeneasVerif/charon/pull/210 is merged and once associated types are lifted to become parameters we should remove this option: https://github.com/AeneasVerif/aeneas/blob/267526a271d5fe04e29fc21a87fbe0f733af33e3/tests/src/traits.rs#L2