[Resources.AWS] Remove unused code

[martincostello]

Changes

Remove redundant check for an ARN prefix spotted while working on #3867.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.95%. Comparing base (129fa90) to head (6bd5757).
⚠️ Report is 21 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3964      +/-   ##
==========================================
- Coverage   71.95%   71.95%   -0.01%     
==========================================
  Files         447      447              
  Lines       17817    17815       -2     
==========================================
- Hits        12820    12818       -2     
  Misses       4997     4997              

Flag	Coverage Δ
unittests-Resources.AWS	74.34% <ø> (-0.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Resources.AWS/AWSECSDetector.cs	88.42% <ø> (-0.24%)	⬇️

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[Copilot]

Pull request overview

Removes a redundant (and effectively dead) ARN-prefix check in the AWS ECS resource detector, simplifying metadata v4 attribute extraction in OpenTelemetry.Resources.AWS.

Changes:

• Deleted an unused clusterArn.StartsWith("arn:") conditional block that computed (but did not use) a baseArn value.

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