[bugfix,parmetis] Remove erroneous undef of HAVE_MPI

This was introduced for debugging only and could have side effects
onto other headers. Removes the accidentally committed line.
[bugfix,parmetis] Always typedef idxtype.

The type idxtype is used by header repartition even if parmetis is not
used. Therefore it has to be typedefed meaningful in this case,
too. This was not the case before and is accomplished by this commit.