RHMWRT-62 and RHOAR-112: Remove ISTIO examples

Missions and Boosters have been renamed to example applications and some of the functionality of the missions and boosters are now provided as capabilities. Other missions and boosters are also not really good as example applications since they are only focusing on showing a particular feature of Istio/Service Mesh.
The following mission should be removed since these are better surfaced as Istio examples and not connected to a particular runtime.

- Istio - Routing
- Istio - Security
- Istio - Circuit Breaker

metadata.yaml

@@ -28,38 +28,6 @@ missions:
   description: Monitor an application's ability to service requests
   metadata:
     level: foundational
-- id: istio-circuit-breaker
-  name: Istio - Circuit Breaker
-  description: The Istio Circuit Breaker mission demonstrates limiting access to a
-    degraded service until it becomes available to handle requests. This helps prevent
-    cascading failures in other services that depend on the failed services for functionality.
-  metadata:
-    level: expert
-    istio: true
-- id: istio-distributed-tracing
-  name: Istio - Distributed Tracing
-  description: The Istio Distributed Tracing mission demonstrates how simple distributed
-    tracing can be integrated with an application running on an Istio-enabled cluster.
-  metadata:
-    level: expert
-    istio: true
-- id: istio-security
-  name: Istio - Security
-  description: Ths Istio Security mission demonstrates how Istio secures communication
-    between microservices with Mutual TLS, and how services can be allowed or denied
-    access to other services using ACL.
-  metadata:
-    level: expert
-    istio: true
-- id: istio-routing
-  name: Istio - Routing
-  description: The Istio Routing mission demonstrates how Istio can be used to route
-    traffic to/from services, including load balancing traffic to different versions
-    of the same service. This can be used to set up A/B tests, canary deployments,
-    or simply do rolling deployments during a production release.
-  metadata:
-    level: expert
-    istio: true
 - id: messaging
   name: Messaging Work Queue
   description: This mission demonstrates how to dispatch tasks to a scalable set of