core: remove default value for runserver cli threads arg

[Castavo]

This allows to use the behavior of Takes to use the max available number of threads, if no threads is given.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 28.94%. Comparing base (a6faf4c) to head (a1ca4fd).
Report is 3 commits behind head on dev.

Files	Patch %	Lines
...c/main/java/fr/sncf/osrd/cli/ApiServerCommand.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #7071      +/-   ##
============================================
- Coverage     28.95%   28.94%   -0.02%     
  Complexity     2244     2244              
============================================
  Files          1070     1070              
  Lines        132543   132543              
  Branches       2713     2714       +1     
============================================
- Hits          38379    38361      -18     
- Misses        92587    92605      +18     
  Partials       1577     1577              

Flag	Coverage Δ
core	79.81% <0.00%> (ø)
editoast	74.77% <ø> (-0.08%)	⬇️
front	9.29% <ø> (ø)
gateway	2.45% <ø> (ø)
railjson_generator	87.15% <ø> (ø)
tests	83.07% <ø> (ø)

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