Remove unnecessary featureflag for eventname

[cijothomas]

Fully fixes #2410 by removing feature flags that are no longer relevant.

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Files not reviewed (1)

• opentelemetry-proto/Cargo.toml: Evaluated as low risk

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.2%. Comparing base (e378bc8) to head (4719b0c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2480   +/-   ##
=====================================
  Coverage   77.2%   77.2%           
=====================================
  Files        124     124           
  Lines      23051   23043    -8     
=====================================
  Hits       17800   17800           
+ Misses      5251    5243    -8     

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[lalitb]

Thanks. LGTM. nit - There is some reference of this flag in stdout exporter too (ref - https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-rust%20populate-logs-event-name&type=code), good to remove it here or separately.