drop mut on some http client usages

thanks to commit 70142e607dda43fc778f39d52dc7bb3bba088cd3 from
proxmox repos's proxmox-http crate

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/tools/subscription.rs

@@ -115,7 +115,7 @@ async fn register_subscription(
         None
     };
 
-    let mut client = pbs_simple_http(proxy_config);
+    let client = pbs_simple_http(proxy_config);
 
     let uri = "https://shop.proxmox.com/modules/servers/licensing/verify.php";
     let query = json_object_to_query(params)?;