Remove legacy TURN credentials request path

[hensmi-amazon]

ℹ️ Description

This path has not been used since 2020.

Issue #, if available

None

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
  • [ ] README update
  • [ ] CHANGELOG update
  • [ ] guides update
• [ ] This change requires a dependency update
  • [ ] Amazon Chime SDK Media
  • [ ] Other (update corresponding legal documents)

🧪 How Has This Been Tested?

Builds. Paths are not currently hit.

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