MINOR: contrib/prometheus-exporter: Remove tests on the option 'http-use-htx'

Configuring the Prometheus service is now always valid for HTTP proxies. So we
don't rely anymore on the flag PR_O2_USE_HTX.
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index 2aafd4d..67914f6 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -2239,10 +2239,6 @@
 		memprintf(err, "Prometheus exporter service only available on 'http-request' rulesets");
 		return ACT_RET_PRS_ERR;
 	}
-	if (!(px->options2 & PR_O2_USE_HTX)) {
-		memprintf(err, "Prometheus exporter service only available for HTX proxies");
-		return ACT_RET_PRS_ERR;
-	}
 
 	/* Add applet pointer in the rule. */
 	rule->applet = promex_applet;