docs(telemetry): drop the inert insight endpoint from the test config template

On the OTel path only [insight] server is load-bearing. CollectorManager reads
endpoint and hands it to OTelCollector, which logs it at startup and routes
nothing with it; the real export endpoint is [telemetry] metrics_endpoint,
which the template already sets. service_instance_id and service_name in that
section are read and discarded.

Leaving the line invited an operator to reconcile a mismatch that has no
effect. integration-test.sh already emits only server=otel with the same
explanation, so the two now agree.

docker/telemetry/TESTING.md

@@ -266,8 +266,10 @@ trace_peer=1
 trace_ledger=1
 
 [insight]
+# server=otel is the only load-bearing key here -- it selects OTelCollector.
+# The export endpoint comes from [telemetry] metrics_endpoint, and [insight]'s
+# own service_instance_id/service_name keys are ignored.
 server=otel
-endpoint=http://localhost:4318/v1/metrics
 
 [rpc_startup]
 { "command": "log_level", "severity": "warning" }