fix(#1189): Unset BACKUP_PROMURL when not explitly set in helm values

[lukasdietrich]

Summary

This pr unsets the global default prometheus url on the operator (BACKUP_PROMURL), unless it is explicitly set in the helm values.

Unless there was a reason for the default of https://127.0.0.1/, this should not be a breaking change.

I created this pr to address #1189.

Checklist

For Helm Chart changes

• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• PR contains the label area:chart
• PR contains the chart label, e.g. chart:k8up
• Commits are signed off
• Variables are documented in the values.yaml using the format required by Helm-Docs.
• Chart Version bumped if immediate release after merging is planned
• I have run make chart-docs
• Link this PR to related code release or other issues.