chore(opentelemetry): remove some unused headers

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request cleans up unused header includes in google/cloud/internal/call_context.h and google/cloud/internal/opentelemetry_context.h, and adds the <utility> header to opentelemetry_context.h. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.35%. Comparing base (0483294) to head (4467920).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16500   +/-   ##
=======================================
  Coverage   92.35%   92.35%           
=======================================
  Files        2258     2258           
  Lines      216448   216448           
=======================================
+ Hits       199899   199901    +2     
+ Misses      16549    16547    -2     

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