helper: Remove unnecessary condition from Comm MPI implementation
helper: Remove unnecessary condition from Comm MPI implementation

The source file is built only when MPI is available, so we do not need
to test for that inside the source.

JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2