cli-hooks(fix): remove environment variable requirements from the start hook

[zimeg]

Summary

This PR removes validation of environment variables from the start hook since it's not always true that these tokens are required to run an app. For instance, an app token isn't needed when running using HTTP Request URLs!

Similar assertions were removed and don't exist in slackapi/python-slack-hooks: slackapi/python-slack-hooks#27

Requirements

• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.81%. Comparing base (9e20ca3) to head (859fe15).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1835      +/-   ##
==========================================
- Coverage   81.85%   81.81%   -0.05%     
==========================================
  Files          35       35              
  Lines        7783     7764      -19     
  Branches      318      318              
==========================================
- Hits         6371     6352      -19     
  Misses       1400     1400              
  Partials       12       12              

Flag	Coverage Δ
cli-hooks	94.94% <ø> (-0.14%)	⬇️
cli-test	54.16% <ø> (ø)
oauth	76.53% <ø> (ø)
socket-mode	59.59% <ø> (ø)
web-api	96.55% <ø> (ø)
webhook	95.27% <ø> (ø)

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[zimeg]

@srajiang thank you for the fast review! I'll merge this now but hold off on a release for a bit more testing on errors and exits 🚀