Remove unused superuser attribute

[frascuchon]

This PR just removes the unused superuser attribute in User pydantic class

[codecov]

Codecov Report

Patch coverage: 92.38% and project coverage change: +0.78 🎉

Comparison is base (8c05058) 92.74% compared to head (0a2f8be) 93.53%.

Additional details and impacted files

@@                         Coverage Diff                         @@
##           feat/add-basic-database-support    #2600      +/-   ##
===================================================================
+ Coverage                            92.74%   93.53%   +0.78%     
===================================================================
  Files                                  167      164       -3     
  Lines                                 8271     8272       +1     
===================================================================
+ Hits                                  7671     7737      +66     
+ Misses                                 600      535      -65     

Flag	Coverage Δ
pytest	93.53% <92.38%> (+0.78%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/argilla/client/api.py	95.34% <ø> (ø)
src/argilla/client/sdk/text_classification/api.py	100.00% <ø> (+12.90%)	⬆️
...rc/argilla/server/daos/backend/mappings/helpers.py	97.22% <ø> (ø)
src/argilla/server/server.py	87.93% <ø> (+0.43%)	⬆️
src/argilla/server/services/datasets.py	92.37% <ø> (-3.51%)	⬇️
src/argilla/server/services/metrics/service.py	100.00% <ø> (ø)
src/argilla/server/services/storage/service.py	100.00% <ø> (ø)
src/argilla/server/settings.py	80.95% <ø> (ø)
src/argilla/tasks/users/create.py	100.00% <ø> (ø)
src/argilla/tasks/users/create_default.py	100.00% <ø> (ø)
... and 36 more

... and 3 files with indirect coverage changes

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