[prometheus-blackbox-exporter] Remove toYaml function call in blackbox exporter extra manifests template

[walker-tom]

What this PR does / why we need it

This allows the resources defined via the extraManifests value to have more 'helm-like' layouts when doing templating within them, such as having the template on the line beneath labels below:

extraManifests:
  - apiVersion: v1
     kind: MyCRD
     metadata:
       labels:
         {{- include "prometheus-blackbox-exporter.labels" . | nindent 4 }}

Which issue this PR fixes

• fixes [prometheus-blackbox-exporter] include in extra-manifests.yaml #4174

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

Thanks!

[walker-tom]

@dpetersen, @Bregor, @redondos or @grobie, do one of you have the time to review this change please? Thanks in advance.