Stop passing --threads to gunicorn

[pjonsson]

This setting only applies to gthread
according to the documentation:

https://docs.gunicorn.org/en/23.0.0/settings.html#threads

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📚 Documentation preview 📚: https://datacube-ows--1061.org.readthedocs.build/en/1061/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.64%. Comparing base (8a86cd3) to head (aa05e6c).
Report is 23 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1061   +/-   ##
=======================================
  Coverage   93.64%   93.64%           
=======================================
  Files          43       43           
  Lines        6510     6510           
=======================================
  Hits         6096     6096           
  Misses        414      414           

[SpacemanPaul]

We actually identified this as an issue about 6 months ago but promptly got bogged down in discussions over whether the correct fix was to remove the --threads argument or to switch to the gthread worker type.

In theory we should be MOSTLY threadsafe. AFAIA the only danger zone is our use of non-re-entrant matplotlib code when auto-generating legend images, which could be manually serialised with a thread lock.

I will use this PR to reinvigorate those discussions, thanks.

Edit: Actually may as well just merge this and open an issue for discussion.

[omad]

Thank you Peter! I removed this foolishness from datacube-explorer, but forgot to follow up with OWS. 🤦🏻