Do not require Prometheus in matrix-prometheus-nginxlog-exporter

The user may be running Prometheus elsewhere. It doesn't need to be
getting installed using the current playbook.

roles/custom/matrix-prometheus-nginxlog-exporter/tasks/validate_config.yml

@@ -1,12 +1,5 @@
 ---
 
-- name: Fail if Prometheus not enabled
-  ansible.builtin.fail:
-    msg: >
-      You need to set `prometheus_enabled: true` or configure an external Prometheus database
-      as this roles requires Prometheus for data storage.
-  when: not prometheus_enabled and not matrix_prometheus_nginxlog_exporter_metrics_proxying_enabled
-
 - name: Fail if docker image not availble for arch
   ansible.builtin.fail:
     msg: >