Clean up `#include`s and `#if HAVE_MPI` in mostly `dune/common/parallel/`
This was meant as a basis for a workaround for OpenMPI's -Wliteral-suffix
annoyance. The workaround did not work out, but I suppose the cleanup is still nice to have.
This should not be too controversial, so I intend to merge in one week (2017-05-19).