As I understand it, once planning permission is granted and that permission implemented there is little or no way to revoke it. So in that sense you might be able to claim the ongoing use of a travel plan is pointless. I'd suggest otherwise, the travel plan had to be created and agreed by the LPA before the permission was effective. It exists as a mechanism to manage any traffic related issues that may occur and it sets out fairly clear statements about the level of car use that should be targeted and the expected impact on the local roads. While deviation from this wouldn't result in planning permission being revoked I'm guessing it could be used to enforce measures to deal with any problems.