Stop enterprise-api panicking on startup from a duplicate /healthz route

main.go registered GET /healthz explicitly, on top of the one
queryapi.Handler.RegisterRoutes already registers -- net/http's
ServeMux panics on a duplicate pattern, so enterprise-api could never
actually start. Every previous "built" claim for this binary had only
ever been a successful go build, never a successful process start;
caught the first time this ran against a real docker-compose stack.

enterprise/cmd/enterprise-api/main.go

@@ -158,9 +158,8 @@ func main() {
 	dashboardsHandler := dashboards.NewHandler(logger, dashboards.NewStore(pgPool), authorizer, rbacstore.NewDashboardPermissions(rbac))
 
 	mux := http.NewServeMux()
-	queryHandler.RegisterRoutes(mux)
+	queryHandler.RegisterRoutes(mux) // also registers GET /healthz
 	dashboardsHandler.RegisterRoutes(mux)
-	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(http.StatusOK) })
 
 	srv := &http.Server{
 		Addr:    cfg.HTTPListenAddr,