fix(mesh-gateway): remove deregistered service from mesh gateway

agent/proxycfg/mesh_gateway.go

@@ -258,6 +258,9 @@ func (s *handlerMeshGateway) handleUpdate(ctx context.Context, u UpdateEvent, sn
 				//                 Do those endpoints get cleaned up some other way?
 				delete(snap.MeshGateway.WatchedServices, sid)
 				cancelFn()
+
+				// always remove the sid from the ServiceGroups when un-watch the service
+				delete(snap.MeshGateway.ServiceGroups, sid)
 			}
 		}
 		snap.MeshGateway.WatchedServicesSet = true

agent/proxycfg/snapshot.go

@@ -482,6 +482,10 @@ func (c *ConfigSnapshot) MeshGatewayValidExportedServices() []structs.ServiceNam
 			continue // not possible
 		}
 
+		if _, ok := c.MeshGateway.ServiceGroups[svc]; !ok {
+			continue // unregistered services
+		}
+
 		chain, ok := c.MeshGateway.DiscoveryChain[svc]
 		if !ok {
 			continue // ignore; not ready