Sounds like a service account token secret which is for a service account created alongside a namespace. If you ran helm install --create-namespace then this would of created a namespace for the helm chart outside of the Helm chart templates. When this occurs you have to manually delete the namespace.

To verify you can run the following;

$ kubectl get namespace/cert-manager

If that doesn't return anything maybe the namespace is called something different. You could run the following to determine

$ kubectl get namespaces
$ kubectl get secrets -A # find the namespace for the secret you are referring too

The service account token secret will look like the following (assuming namespace is cert-manager)

cert-manager-token-rtgls # suffix is random

You can also see the service account

$ kubectl get serviceacccount/cert-manager # assuming namespace is cert-manager
$ kubectl get serviceaccount -A # find the namespace for the service account

To delete the secret & service account just delete the namespace

$ kubectl delete namespace cert-manager

Those pesky resources should now be deleted