Don't transform mesh collision elements twice.

This fixes a sizable bug in our handling of mesh collision elements. The element-to-link transform for `RigidBodyMesh` objects was being applied twice - once in `RigidBodyMesh/getPoints` and again in  `DrakeCollision::BulletElement`. This pull request sets the element-to-link transform to identity for mesh collision elements in `constructModelmex`, since the points returned by `RigidBodyMesh/getPoints` are already in the link frame.