Remove conversion configuration for ClusterTask

`ClusterTask` only exists in `v1beta1` and will never have a `v1` thus
doesn't need to be registered in the conversion webhook.

Prior to this commit, some tooling (like
https://github.com/openshift/cluster-kube-apiserver-operator/) might
assume there is a conversion webhook and then fail to communicate with
it (because we do not register `ClusterTask` in the conversion webhook
part) ; generating a lot of spam log.

As a rules of thumb, if we are not registering/refering an object in
`newConversionController` (in `cmd/webhook/main.go`), the `conversion`
field in the CRD shouldn't be defined either.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

config/300-crds/300-clustertask.yaml

@@ -51,11 +51,3 @@ spec:
     - tekton
     - tekton-pipelines
   scope: Cluster
-  conversion:
-    strategy: Webhook
-    webhook:
-      conversionReviewVersions: ["v1beta1"]
-      clientConfig:
-        service:
-          name: tekton-pipelines-webhook
-          namespace: tekton-pipelines