ci: rework E2E infrastructure to use default namespace

Board directive: E2E tests must run in the `default` namespace.
Nothing should persist beyond a test run; no dedicated namespace needed.

Changes:
- e2e-ci-runner-rbac.yaml: retarget Role/RoleBinding to `default`,
  remove ClusterRole/ClusterRoleBinding (no longer needed since we
  don't need cluster-scoped namespace read permission)
- e2e.yaml: set E2E_NAMESPACE=default
- deploy-e2e-headlamp.sh: default namespace to `default`, remove
  namespace existence check (default always exists)
- teardown-e2e-headlamp.sh: default namespace to `default`, remove
  namespace existence check guard
- headlamp-e2e-values.yaml: update usage comment
- e2e/README.md: remove namespace creation prerequisite

Closes #78 #79

Co-Authored-By: Paperclip <noreply@paperclip.ing>

e2e/README.md

@@ -275,7 +275,6 @@ No custom Docker images, no PVCs, no kubectl exec/cp, no patching of existing de
 One-time setup by a cluster admin:
 
 ```bash
-kubectl create namespace headlamp-e2e
 kubectl apply -f deployment/e2e-ci-runner-rbac.yaml
 ```