test(webpack-4): remove dev server dependency and start script

[pichlermarc]

Which problem is this PR solving?

See #6103 - there's a vulnerability in webpack-dev-server@4. However we only use it in a script that does not really have a purpose for the current test setup - so it's unused code that can be easily removed to make the alerts go away.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.16%. Comparing base (9849536) to head (9909abf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6120   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files         316      316           
  Lines        9226     9226           
  Branches     2082     2082           
=======================================
  Hits         8780     8780           
  Misses        446      446           

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[raphael-theriault-swi]

Makes sense to me