removed fields which are added by extensions from the base Search mod…

[rsmith013]

…el. closes #99

[codecov-commenter]

Codecov Report

Merging #100 (9679f27) into master (7c119da) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #100   +/-   ##
=======================================
  Coverage   96.27%   96.27%           
=======================================
  Files          20       20           
  Lines         483      483           
=======================================
  Hits          465      465           
  Misses         18       18           

Flag	Coverage Δ
unittests	96.27% <100.00%> (ø)

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Impacted Files	Coverage Δ
stac_pydantic/item.py	93.87% <ø> (-0.13%)	⬇️
stac_pydantic/api/search.py	96.20% <100.00%> (+0.04%)	⬆️

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