Drop prometheus-exporter for intermal metrics

[wrenix]

See server PRs:

• feat(openmetrics): introduce OpenMetrics exporter server#57165
• fix(openmetrics): Multiple exporters per app server#57765
• fix(openmetrics): Allow openmetrics in info.xml server#57781

Documentation is here:
https://docs.nextcloud.com/server/latest/admin_manual/configuration_monitoring/index.html