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

tests/unit/distribution/test_distribution.py

@@ -196,8 +196,6 @@ class TestProviderRegistry:
             assert internal_api not in apis, f"Internal API {internal_api} should not be in providable_apis"
 
         for api in apis:
-            if api == Api.telemetry:
-                continue
             module_name = f"llama_stack.providers.registry.{api.name.lower()}"
             try:
                 importlib.import_module(module_name)