Remove extra-config for cert-manager (#2708)

Minikube does the right thing (as of 0.28.1 at least) with creating the embedded CA.  The extra-config parameters appear to have been necessary previously and were resolved to use the "right" credentials built by Minikube directly. In fact, passing those parameters appears to break current minikube deployments, making it impossible to create new service accounts and resources that rely on them. (like a tiller service account for a helm deployment of Istio...)

I found this bug that referenced this issue: https://github.com/kubernetes/minikube/issues/1647 which is now closed.

content/docs/setup/kubernetes/platform-setup/minikube/index.md

@@ -9,7 +9,7 @@ keywords: [platform-setup,kubernetes,minikube]
 Follow these instructions to prepare Minikube for Istio.
 
 1. To run Istio locally, install the latest version of
-   [Minikube](https://kubernetes.io/docs/setup/minikube/), version **0.28.0 or
+   [Minikube](https://kubernetes.io/docs/setup/minikube/), version **0.28.1 or
    later**.
 
 1. Select a
@@ -23,8 +23,6 @@ Follow these instructions to prepare Minikube for Istio.
 
     {{< text bash >}}
     $ minikube start --memory=8192 --cpus=4 --kubernetes-version=v1.9.4 \
-        --extra-config=controller-manager.cluster-signing-cert-file="/var/lib/localkube/certs/ca.crt" \
-        --extra-config=controller-manager.cluster-signing-key-file="/var/lib/localkube/certs/ca.key" \
         --extra-config=apiserver.admission-control="NamespaceLifecycle,LimitRanger,ServiceAccount,PersistentVolumeLabel,DefaultStorageClass,DefaultTolerationSeconds,MutatingAdmissionWebhook,ValidatingAdmissionWebhook,ResourceQuota" \
         --vm-driver=`your_vm_driver_choice`
     {{< /text >}}
@@ -33,7 +31,5 @@ Follow these instructions to prepare Minikube for Istio.
 
     {{< text bash >}}
     $ minikube start --memory=8192 --cpus=4 --kubernetes-version=v1.10.0 \
-        --extra-config=controller-manager.cluster-signing-cert-file="/var/lib/localkube/certs/ca.crt" \
-        --extra-config=controller-manager.cluster-signing-key-file="/var/lib/localkube/certs/ca.key" \
         --vm-driver=`your_vm_driver_choice`
     {{< /text >}}