Delete deprecated and unused methods from distribute_lib. These methods

all had the `@doc_controls.do_not_generate_docs` decorator since we knew
they were going away as soon as references to them were updated to point
to the official APIs decided in the accepted RFC:
https://github.com/tensorflow/community/blob/master/rfcs/20181016-replicator.md

PiperOrigin-RevId: 229398974