Don't try to suppress use of MPI C++ binding

This doesn't really work because we don't currently
propagate the necessary compile flags, and CMake tries
to link against the mpi_cxx library anyway.
Instead, we'll set the necessary config globally in our
build system to build RHEL packages that also work
on Fedora 40 or later (where the C++ bindings are gone).

This reverts commit 7bfed8c.
This reverts commit 841a81d.

modules/mpi/dependency/MPI.cmake

@@ -1,7 +1,3 @@
-# We don't need the deprecated C++ bindings, so try to suppress linkage
-# against them
-set(MPI_CXX_SKIP_MPICXX TRUE CACHE BOOL "" FORCE)
-
 find_package(MPI)
 
 if("${MPI_CXX_FOUND}")