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
• 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.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[seratch]

Thank you!