remove use of MPI_COMM_WORLD

[cyrush]

use comm which is available in the class instance:

conduit/src/libs/blueprint/conduit_blueprint_mpi_mesh_flatten.cpp

Line 164 in ad86e31

DEBUG_PRINT("Rank " << relay::mpi::rank(MPI_COMM_WORLD) << " - from_node" << std::endl);

[cyrush]

note, this is debugging code and isn't active, so I don't expect a hang in the wild, but it would still be good to fix