Timesurface hacks

Micro-modification on TimeSurface.

- Remove unused variable.
- Remove deprecated TODO comment (related to the changes made to the accumulative sampler to decorrelate off-road distance and cell size.)
- Make a method private in SurfaceResource.

I think it could be nice in the future to be able to create a sample grid from any source (SPT or int[] array or whatever that gives a value for a vertex), and to allow a TimeSurface to be more generic: either storing a sample value for each vertex or a sampled surface. If the TimeSurface public interface is "getTime(Vertex)" this should be feasible.