[!345] Get rid of the UGGrid::heapSize_ data member

Merge branch 'get-rid-of-uggrid-heapsize' into 'master'

ref:core/dune-grid \[This is WIP because it needs some minor fixes in
dune-uggrid.\]

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.

See merge request [!345]

  [!345]: gitlab.dune-project.org/core/dune-grid/merge_requests/345