remove all unneeded schizo framework components

[hppritcha]

As part of the integration process of this PRRTe fork into Open MPI, the PRRTe schizo framework can be simplified to only support Open MPI. Eventually the framework can be removed entirely, but this is a useful first step.

The components that can be removed are:

• https://github.com/open-mpi/prrte/tree/master/src/mca/schizo/mpich
• https://github.com/open-mpi/prrte/tree/master/src/mca/schizo/slurm