manifests: remove prometheus.io/scrape annotations

These annotations made sense in pre v1.3.0 Prometheus releases, however,
with >=v1.3.0 and the Prometheus Operator these annotations are more
confusing then helpful.

manifests/examples/example-app/example-app.yaml

@@ -4,8 +4,6 @@ metadata:
   name: example-app
   labels:
     tier: frontend
-  annotations:
-    prometheus.io/scrape: 'true'
 spec: 
   selector: 
     app: example-app