chore: Removed langchain_instrumentation feature flag as the instrumentation is feature complete

[bizob2828]

Description

langchain is feature complete. feature flag is no longer needed.

Related Issues

Closes #2072

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.18%. Comparing base (66f94b0) to head (eebad7f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2073      +/-   ##
==========================================
- Coverage   97.18%   97.18%   -0.01%     
==========================================
  Files         248      248              
  Lines       41938    41936       -2     
==========================================
- Hits        40756    40754       -2     
  Misses       1182     1182              

Flag	Coverage Δ
integration-tests-16.x	78.23% <75.00%> (+<0.01%)	⬆️
integration-tests-18.x	78.22% <75.00%> (-0.01%)	⬇️
integration-tests-20.x	78.23% <75.00%> (+<0.01%)	⬆️
unit-tests-16.x	90.49% <100.00%> (-0.01%)	⬇️
unit-tests-18.x	90.47% <100.00%> (-0.01%)	⬇️
unit-tests-20.x	90.47% <100.00%> (-0.01%)	⬇️
versioned-tests-16.x	74.49% <75.00%> (-0.02%)	⬇️
versioned-tests-18.x	75.60% <100.00%> (-0.03%)	⬇️
versioned-tests-20.x	75.61% <100.00%> (-0.03%)	⬇️

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