remove unused multiprocessing test mode

[WilliamBergamin]

This PR aims to remove the logic that implements the multiprocessing testing mode (in favor of the threading mode). This mode is no longer used in CI/CD or development.

This PR paves the path to implement similar changes brought to the slack-sdk in #1445 that replaces the Flask-Sockets dependency with aiohttp

Category

• [ ] slack_bolt.App and/or its core components
• [ ] slack_bolt.async_app.AsyncApp and/or its core components
• [ ] Adapters in slack_bolt.adapter
• [ ] Document pages under /docs
• [x] Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5e63905) 91.76% compared to head (1b86be7) 91.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1011 +/- ## ======================================= Coverage 91.76% 91.76% ======================================= Files 181 181 Lines 6312 6312 ======================================= Hits 5792 5792 Misses 520 520 ```

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