[prometheus-blackbox-exporter] Remove toYaml from extraManifests template

[iandrewt]

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 }}

What this PR does / why we need it

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

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

Special notes for your reviewer

This is a repeat of #4220, which was abandoned.

Checklist

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