UGGrid: Remove the rudimentary saveState/loadState implementations

The implementations of these methods looked like they do something reasonable, but in fact they don't: They do call corresponding UGGrid methods, but those methods never worked (in Dune!).

After giving it some thought it is now my opinion that the UGGrid saveState/loadState functionality is not worth the time needed to understand it and get it to work. A modern snapshotting code should be written from scratch in any case, using existing modern standards as much as possible.

Therefore, this patch removes the dummy calls to SaveMultiGrid and LoadMultiGrid.