Remove duplicate MPI launcher abstractions

[benclifford]

Is your feature request related to a problem? Please describe.

PR #3016 introduced a second MPI launcher abstraction inside the High Throughput Executor, distinct from the long-existing parsl.launchers abstraction.

Mostly this was driven by the new usages of launchers in #3016, although extremely similar to earlier usage, being different enough that the existing launcher API could not support this new use.

The launcher abstraction should be fixed to support all these use cases.

Describe the solution you'd like

one launcher abstraction

Describe alternatives you've considered

status quo: forcing all future launcher implementers, maintainers and users to deal with two MPI launcher abstractions

Additional context

In #3016 (review) I merged #3016 with severe reservations including the subject of this current issue. This issue is to turn that in-passing comment into a numbered issue.