[linear][parallelhelper] remove obsolete mpi guard

On minimal setups without MPI installed, the box tests failed to compile
as the FVAssembler uses the ParallelVectorHelper class, which itself was
hidden behind a HAVE_MPI guard. Tests with setups where MPI is not
installed showed that the tests compile and run also without the guard.

dumux/linear/parallelhelpers.hh

@@ -25,8 +25,6 @@
 #ifndef DUMUX_LINEAR_PARALLELHELPERS_HH
 #define DUMUX_LINEAR_PARALLELHELPERS_HH
 
-#if HAVE_MPI
-
 #include <dune/geometry/dimension.hh>
 #include <dune/grid/common/datahandleif.hh>
 #include <dune/grid/common/partitionset.hh>
@@ -788,5 +786,4 @@ void prepareLinearAlgebraParallel(Matrix& A, Vector& b,
 
 } // end namespace Dumux
 
-#endif // HAVE_MPI
 #endif