Do not add main/master Synapse process to instance_map if workers are not in use

This is meant to keep the configuration tidy for non-worker setups.

Related to 49cb8b7b11

roles/custom/matrix-synapse/defaults/main.yml

@@ -721,7 +721,7 @@ matrix_synapse_instance_map: |
         'host': 'matrix-synapse',
         'port': matrix_synapse_replication_http_port,
       },
-    }
+    } if matrix_synapse_workers_enabled else {}
   }}
 
 # Redis information