libpmi: drop pmix client support and --with-pmix configure option

[garlick]

pmix code added to flux for sierra was not especially helpful, and caused a few problems. As discussed in #1662, this PR backs it out.

We'll need some RPM on that architecture to provide libpmix (with libpmi.so) that interoperates with spectrum MPI's built-in version, by the time we roll out the next release of Flux on sierra.

[grondo]

This seems wise, as I think the --with-pmix option also caused some confusion for users.

Thanks! I'm fine with merging, but we should probably get at least one more ack first.

[SteVwonder]

Per @trws's suggestion in #1662:

Should we add a recommended dep or similar on an external PMIX with PMI?

Can this PR also update the readme to specify that we require either a PMI libary or a PMIx library with PMI compatibility?

[garlick]

Good idea, I'll tack on an update to README.md.

[coveralls]

Coverage increased (+0.5%) to 79.931% when pulling 773c10b on garlick:nix_pmix into 7c38aa3 on flux-framework:master.

[grondo]

Thanks @garlick!