Initialize ParallelIndexSet::deletedEntries_ to fix Clang sanatize runs
Otherwise assignment of a newly created ParallelIndexSet will provoke an error because of access to uninitialized values when compiled with -fsanitize=undefined:
/usr/include/dune/common/parallel/indexset.hh:216:9: runtime error: load of value 192, which is not a valid value for type 'bool'