Remove redundant asyncio marks in tests and fixtures

[jotare]

Description

pytest's asyncio_mode is always set to auto. This means all async fixtures and tests are automatically annotated with pytest.mark.asyncio and we don't need to. In addition, fixtures annotated with this mark throw a noisy error on every test execution.

How was this PR tested?

Existing test suite

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.83%. Comparing base (dac174f) to head (03e811c).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2736      +/-   ##
==========================================
+ Coverage   86.76%   86.83%   +0.07%     
==========================================
  Files         383      383              
  Lines       24184    24200      +16     
==========================================
+ Hits        20983    21014      +31     
+ Misses       3201     3186      -15     

Flag	Coverage Δ
nucliadb	74.54% <ø> (+0.07%)	⬆️
nucliadb-ingest	28.70% <ø> (ø)
nucliadb-reader	24.60% <ø> (ø)
nucliadb-search	38.06% <ø> (ø)
nucliadb-standalone	46.89% <ø> (ø)
nucliadb-train	45.45% <ø> (-0.03%)	⬇️
nucliadb-writer	38.98% <ø> (ø)
nucliadb_dataset	55.45% <ø> (ø)
nucliadb_models	83.42% <ø> (ø)
nucliadb_sdk	80.11% <ø> (ø)
nucliadb_sidecar	89.03% <ø> (ø)
nucliadb_telemetry	86.56% <ø> (ø)
nucliadb_utils	83.93% <ø> (ø)

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