chore(telemetry): code cleanup (#3897)

# What does this PR do?
Clean up telemetry code since the telemetry API has been remove.
- moved telemetry files out of providers to core
- removed from Api

## Test Plan

❯ OTEL_SERVICE_NAME=llama_stack
OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318 uv run llama stack run
starter
❯ curl http://localhost:8321/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-4o-mini",
    "messages": [
      {
        "role": "user",
        "content": "Hello!"
      }
    ]
  }'

-> verify traces in Grafana

CI

llama_stack/providers/utils/inference/litellm_openai_mixin.py

@@ -256,7 +256,7 @@ class LiteLLMOpenAIMixin(
         params: OpenAIChatCompletionRequestWithExtraBody,
     ) -> OpenAIChatCompletion | AsyncIterator[OpenAIChatCompletionChunk]:
         # Add usage tracking for streaming when telemetry is active
-        from llama_stack.providers.utils.telemetry.tracing import get_current_span
+        from llama_stack.core.telemetry.tracing import get_current_span
 
         stream_options = params.stream_options
         if params.stream and get_current_span() is not None: