Revert "Allow setting custom server and key with env variables"

src/ui_interface.rs

@@ -243,11 +243,7 @@ pub fn set_peer_option(id: String, name: String, value: String) {
 
 #[inline]
 pub fn using_public_server() -> bool {
-    if hbb_common::config::RS_PUB_KEY == hbb_common::config::RS_DEF_PUB_KEY {
-        return true
-    } else {
-        return false
-    }
+    crate::get_custom_rendezvous_server(get_option_("custom-rendezvous-server")).is_empty()
 }
 
 #[inline]