Remove wrong claim about naturality law.

This docs change removes a longstanding confusion in the Traversable docs. The docs say "(The naturality law is implied by parametricity and thus so is the purity law [1, p15].)". However if one reads the reference a different "natural" law is implied by parametricity. The naturality law given as a law here is imposed. Further, the reference gives examples which violate both laws -- so they cannot be implied by parametricity. This PR just removes the wrong claim.