Hardcode prometheus as user and group name

Not much sense or need in using variables here. The systemd unit file
was using hard-coded values anyway.

roles/prometheus/defaults/main.yml

@@ -1,7 +1,5 @@
 ---
 prometheus_version: 3.2.1
 prometheus_dir: "/etc/prometheus"
-prometheus_user: prometheus
-prometheus_group: prometheus
 prometheus_config_dir: "/etc/prometheus/conf"
 prometheus_db_dir: "/var/lib/prometheus"