Disable ParMetis support in the examples

ParMetis does something weird with the mpi.h header and C linkage.
This leads to "conflicting declaration" errors for MPI methods.
As we are not using ParMetis anyway let's just disable it.

Yes, this is not particularly pretty, but the problem will go away
once we port away from dune-pdelab, which we plan on doing anyway.
10 jobs for import-examples in 25 minutes and 31 seconds (queued for 2 seconds)