Describe the bug:
After successfully completing dns-01 challenges, cert-manager is not always cleaning up the TXT records it created

Expected behaviour:
All DNS records related to challenges should be deleted once completed.

Steps to reproduce the bug:
TBC.

I currently cannot consistently reproduce the issue

Anything else we need to know?:

Environment details::

• Kubernetes version: v1.17.14-gke.1600
• Cloud-provider/provisioner: GKE
• cert-manager version: 1.1.0
• Install method: Custom helm chart

The issue only seems to affect challenge records provisioned in Google Cloud DNS, we don't see the same thing for Cloudflare DNS (Though about 95% of challenges are via Cloud DNS)

I can see in the GCP logging for one example the API requests to create the record, but no requests to later delete the record.

/kind bug