[prometheus-node-exporter] Remove toYaml function in extraManifests template

[shoce]

@gianrubio @zanhsieh @zeritti

Analogous to #3287 made for prometheus chart

What this PR does / why we need it

Trying to add an extra manifest

extraManifests:
  - |
      apiVersion: v1
      kind: ConfigMap
      metadata:
        namespace: prometheus-node-exporter
        name: configmap-prometheus-node-exporter-web-yaml
      data:
        prometheus-node-exporter-web.yaml: |
          # https://github.com/prometheus/exporter-toolkit/blob/master/docs/web-configuration.md
          basic_auth_users:
            admin: $aa$hashedpassword

getting helm install error:

Error: UPGRADE FAILED: YAML parse error on prometheus-node-exporter/templates/extra-manifests.yaml: error unmarshaling JSON: while decoding JSON: json: cannot unmarshal string into Go value of type releaseutil.SimpleHead

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name

[zeritti]

Thank you, @shoce, for your PR. Please, update the values file indicating the correct usage in the given example.

[shoce]

Thank you, @shoce, for your PR. Please, update the values file indicating the correct usage in the given example

done