Remove Consul from Praefect diagram

Initially we will rely on PostgreSQL for leader election rather than
Consul. This leaves the option open for later, and also leaves options
open for cloud native approaches in Kubernetes.

doc/administration/gitaly/praefect.md

@@ -28,7 +28,7 @@ reference architecture additionally requires:
 - 1 PostgreSQL server (PostgreSQL 9.6 or newer)
 - 3 Gitaly nodes (1 primary, 2 secondary)
 
-![Alpha architecture diagram](img/praefect_architecture_v12_9.png)
+![Alpha architecture diagram](img/praefect_architecture_v12_10.png)
 
 See the [design
 document](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/design_ha.md)