net/openmpi: revert previous commit

The sysutils/slurm-wlm dependency has been removed purposely, because
OpenMPI does not require Slurm to be present at build time in order to
enable Slurm support. The Slurm integration in Open MPI is handled
through the PMIx interface, not via direct linkage to Slurm itself.

For reference, see the Open MPI documentation on PMIx:
https://docs.open-mpi.org/en/v5.0.0/launching-apps/pmix-and-prrte.html#pmix

"PMIx presents a unified API that hides many of the complexities of
communication with these back-end run-time environments. Open MPI uses
the PMIx API to discover, communicate, and coordinate with any supported
back-end run-time system without needing to know the intimate details of
that system."