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)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #115565

00:03:17

passed dune:git clang C++17 with examples #115551

00:09:44

passed dune:git gcc-8 C++17 #115564

00:05:15

passed dune:git gcc-8 C++17 with examples #115552

00:10:31

failed dune:git clang C++17 #115549

00:24:51

failed dune:git clang C++17 #115296

00:01:03

failed dune:git clang C++17 with examples #115298

00:01:44

failed dune:git gcc-8 C++17 #115550

01:00:30

failed dune:git gcc-8 C++17 #115297

00:02:52

failed dune:git gcc-8 C++17 with examples #115299

00:06:50