Remove unnecessary endpoint logic, rename collaborative to hivemind

[SeanNaren]

What does this PR do?

After the introduction of the DHT CLI in hivemind, there isn't the need for the endpoint anymore. Also missed a few renames from #13388.

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cc @Borda

[akihironitta]

The related doc filenames should also be renamed to hivemind, maybe?

occurrences

$ grep -rnI collaborative docs
docs/source-pytorch/index.rst:206:   Training over the internet <strategies/collaborative_training>
docs/source-pytorch/index.rst:250:   Collaborative Training <strategies/collaborative_training>
docs/source-pytorch/strategies/collaborative_training.rst:1:.. _collaborative_training:
docs/source-pytorch/strategies/collaborative_training.rst:20:   :button_link: collaborative_training_basic.html
docs/source-pytorch/strategies/collaborative_training.rst:28:   :button_link: collaborative_training_intermediate.html
docs/source-pytorch/strategies/collaborative_training.rst:36:   :button_link: collaborative_training_expert.html
docs/source-pytorch/strategies/collaborative_training_expert.rst:3:.. _collaborative_training_expert:
docs/source-pytorch/strategies/collaborative_training_expert.rst:11:Below are some ways to reduce communication when training collaboratively. As the size of your model increase, bottlenecks in communication become more apparent.
docs/source-pytorch/strategies/collaborative_training_intermediate.rst:3:.. _collaborative_training_intermediate:
docs/source-pytorch/strategies/collaborative_training_basic.rst:3:.. _collaborative_training_basic:
docs/source-pytorch/extensions/strategy.rst:78:   * - collaborative
docs/source-pytorch/extensions/strategy.rst:80:     - Strategy for training collaboratively on local machines or unreliable GPUs across the internet. :ref:`Learn more. <strategies/collaborative_training:Training on unreliable mixed GPUs across the internet>`
docs/source-pytorch/expertise_levels.rst:82:Learn to scale up your models and enable collaborative model development at academic or industry research labs.
docs/source-pytorch/common_usecases.rst:130:   :button_link: strategies/collaborative_training
docs/source-pytorch/levels/intermediate.rst:6:Learn to scale up your models and enable collaborative model development at academic or industry research labs.