chore(telemetry): remove service_name entirely (#1755)

# What does this PR do?


## Test Plan

LLAMA_STACK_CONFIG=dev pytest -s -v
tests/integration/agents/test_agents.py::test_custom_tool
--safety-shield meta-llama/Llama-Guard-3-8B --text-model
accounts/fireworks/models/llama-v3p1-8b-instruct

and verify trace in jaeger UI
https://llama-stack.readthedocs.io/en/latest/building_applications/telemetry.html#

llama_stack/providers/inline/telemetry/meta_reference/config.py

@@ -28,11 +28,6 @@ class TelemetryConfig(BaseModel):
         default="http://localhost:4318/v1/metrics",
         description="The OpenTelemetry collector endpoint URL for metrics",
     )
-    service_name: str = Field(
-        # service name is always the same, use zero-width space to avoid clutter
-        default="​",
-        description="The service name to use for telemetry",
-    )
     sinks: List[TelemetrySink] = Field(
         default=[TelemetrySink.CONSOLE, TelemetrySink.SQLITE],
         description="List of telemetry sinks to enable (possible values: otel, sqlite, console)",
@@ -52,7 +47,6 @@ class TelemetryConfig(BaseModel):
     @classmethod
     def sample_run_config(cls, __distro_dir__: str, db_name: str = "trace_store.db") -> Dict[str, Any]:
         return {
-            "service_name": "${env.OTEL_SERVICE_NAME:​}",
             "sinks": "${env.TELEMETRY_SINKS:console,sqlite}",
             "sqlite_db_path": "${env.SQLITE_DB_PATH:" + __distro_dir__ + "/" + db_name + "}",
         }