Stop using deprecated variable name (prometheus_node_exporter_server_fqn)

group_vars/matrix_servers

@@ -3859,9 +3859,7 @@ prometheus_node_exporter_base_path: "{{ matrix_base_data_path }}/prometheus-node
 prometheus_node_exporter_uid: "{{ matrix_user_uid }}"
 prometheus_node_exporter_gid: "{{ matrix_user_gid }}"
 
-# _server_fqn is the old var, _hostname - the new one. Seamless migration
-prometheus_node_exporter_server_fqn: "{{ matrix_server_fqn_matrix }}"
-prometheus_node_exporter_hostname: "{{ prometheus_node_exporter_server_fqn }}"
+prometheus_node_exporter_hostname: "{{ matrix_server_fqn_matrix }}"
 
 prometheus_node_exporter_container_network: "{{ matrix_docker_network }}"