Get rid of the UGGrid::heapSize_ data member

[This is WIP because it needs some minor fixes in dune-uggrid.]

The hand-coded memory heap of UG has been removed from dune-uggrid about a year ago. Hence, the heapSize parameter of UGGrid does not have any purpose anymore. This patch removes it completely. It only leaves the public method setDefaultHeapSize and marks it as deprecated, in order not to break people's codes.