Remove some MPI-2.1 checks
Remove checking of these MPI-2.1 (2008) functions and their PETSC_HAVE_XXX macro (if any)
- MPI_Exscan
- MPI_Reduce_scatter
- MPI_Type_get_envelope
- MPI_Finalized
- MPI_Type_dup
As this user report https://lists.mcs.anl.gov/pipermail/petsc-users/2022-March/045771.html implies, they bring unnecessary burdens than warranted.
I left PETSC_HAVE_MPI_INIT_THREAD
unchanged as we do not need MPI thread.
Edited by Junchao Zhang