remove cert-manager-clusterissuer

Ref: #31 (comment 8116142)

templates/cert-manager-issuer.yaml

-apiVersion: cert-manager.io/v1
-kind: Issuer
-metadata:
-  name: letsencrypt
-  namespace: {{ .Release.namespace }}
-spec:
-  acme:
-    # The ACME server URL
-    server: {{ .Values.cern.tls.acme.server }}
-    # Email address used for ACME registration
-    email: {{ .Values.cern.tls.acme.email }}
-    # Name of a secret used to store the ACME account private key
-    privateKeySecretRef:
-      name: letsencrypt
-    # Enable the HTTP-01 challenge provider
-    solvers:
-    - http01:
-        ingress:
-          class: nginx