Remove dual stack observability sample

At the moment, AgentCore Observability does not support "dual stack observability" for sending traces to multiple targets. This sample needs to be removed until support is added to avoid confusion. Builders who like to send observability logs to 3rd party, can do so by disabling AgentCore Observability and configuring 3rd party observability tools.

**Sample to be removed:** https://github.com/awslabs/amazon-bedrock-agentcore-samples/tree/main/03-integrations/observability/simple-dual-observability
**Alternative 3P Observability integration samples:** https://github.com/awslabs/amazon-bedrock-agentcore-samples/tree/main/01-tutorials/06-AgentCore-observability/04-Agentcore-runtime-partner-observability