Remove realtime settings updates endpoint

[marksmith]

This change removes the Aikido::Zen.config.realtime_settings_updates_endpoint and the associated probing machinery, used to probe whether the realtime settings updates endpoint is accessible and use it for polling.

This is in preparation for an ongoing simplification of endpoints used by Zen by Aikido and should not be merged until the unified realtime endpoint supports realtime settings updates, otherwise this will not work, even if the feature flag is enabled.

See #285 for more information about realtime software updates.

Summary by Aikido

Security Issues: 0	Quality Issues: 0	Resolved Issues: 0

🔧 Refactors

• Removed realtime settings updates endpoint configuration and probe
• Simplified agent startup to always enable API stream without probing
• Replaced APIStream probing with direct use of realtime endpoint

^{More info}