[AlbertaGrid] [cleanup] include dune/grid/common/entityiterator.hh

Originally, there were three different facade classes for the different
iterators of entities: LeafIterator, LevelIterator, HiearchicIterator.
They all have been combined into the new EntityIterator facade. This
patch removes the old includes for the corresponding facades in favor of
the dune/grid/common/entityiterator.hh.