fix: remove premature MPI environment variable check in OpenMPIRunner

[leejianwoo-collab]

fix: remove premature MPI environment variable check in OpenMPIRunner

Resolves issue #7711 where OpenMPI launcher incorrectly checked for MPI
environment variables before mpirun execution.

The problematic check in validate_args() was removed because:

• MPI environment variables (OMPI_COMM_WORLD_LOCAL_RANK, OMPI_COMM_WORLD_RANK,
  OMPI_COMM_WORLD_SIZE) are set by mpirun, not before it runs
• This timing issue caused false "MPI environment variables are not set" errors
• The _setup_mpi_environment() method is preserved for later use when appropriate

This fix allows OpenMPI launcher to work correctly without requiring users to
comment out the validation manually.

[tohtana]

Hi @leejianwoo-collab,
Thank you for the fix! It was the only usage of _setup_mpi_environment other than tests. Thus, I think we should remove the function itself and the related tests.
I opened a PR to remove the code that will be unnecessary: leejianwoo-collab#1
Can you merge it?