otel-agent: let rbac be the only place the identity is named

Chart 0.1.11 derives the pod's ServiceAccount from the rbac block, so stating
it a second time at the top level says nothing the declaration did not already
say -- and two places to write one name is how they came to disagree in the
first place.

The chart now fails the render if they ever do disagree, so this is safe to
carry as one line instead of two.

deploy/zoo-system/otel-agent.yaml

@@ -71,13 +71,6 @@ env:
         fieldPath: status.hostIP
   - name: OTEL_RESOURCE_ATTRIBUTES
     value: k8s.node.name=$(K8S_NODE_NAME)
-# ATTACHES the identity below. rbac.serviceAccountName CREATES a
-# ServiceAccount and binds the roles to it; this is what makes the POD run as
-# it. Setting only the rbac key leaves the pod on `default`, which has none of
-# the permissions — and the failure is a 403 from the kubelet, not a warning
-# from the chart. That is exactly how kubeletstats silently returned nothing on
-# this cluster while hostmetrics worked fine.
-serviceAccountName: otel-agent
 rbac:
   create: true
   serviceAccountName: otel-agent