Remove hardcoded streaming URLs from demos and tests

[untoldengine]

Drop the remote-scene streaming options from ShowcaseDemo and LargeSceneStreamingDemo that pointed at public CloudFront manifests, and disable RemoteStreamFlyThroughTests until a replacement manifest host is available.

Summary

Checklist

This project follows the Engineering Principles.
Check what applies to this change; it's fine to leave an item unchecked if it
genuinely doesn't apply.

• Relevant tests have been added or updated
• Existing functionality and API compatibility have been considered
• Relevant builds and tests pass locally
• Recoverable failures are handled appropriately (no silent failures, no unnecessary crashes)
• Significant architectural changes and trade-offs are explained below
• Documentation has been updated where necessary
• Performance-sensitive changes have been measured (profiling/benchmarks), if applicable

Architectural notes / trade-offs