Do not fail on unrelated validation tasks when Prometheus not enabled

These validation tasks should only run when Prometheus is enabled.

roles/matrix-prometheus-node-exporter/tasks/validate_config.yml

@@ -1,7 +0,0 @@
----
-
-- name: Fail if Synapse metrics or Prometheus Node Exporter not enabled
-  fail:
-    msg: >
-      You need to enable `matrix_synapse_metrics_enabled` and/or `matrix_prometheus_node_exporter_enabled` for Prometheus grab metrics.
-  when: "not matrix_synapse_metrics_enabled and not matrix_prometheus_node_exporter_enabled"

roles/matrix-prometheus/tasks/main.yml

@@ -3,7 +3,7 @@
     - always
 
 - import_tasks: "{{ role_path }}/tasks/validate_config.yml"
-  when: run_setup|bool
+  when: "run_setup|bool and matrix_prometheus_enabled|bool"
   tags:
     - setup-all
     - setup-prometheus