feat: do not set "unknown sender for this chat" error

src/stock_str.rs

@@ -123,9 +123,6 @@ pub enum StockMessage {
                     however, of course, if they like, you may point them to 👉 https://get.delta.chat"))]
     WelcomeMessage = 71,
 
-    #[strum(props(fallback = "Unknown sender for this chat."))]
-    UnknownSenderForChat = 72,
-
     #[strum(props(fallback = "Message from %1$s"))]
     SubjectForNewContact = 73,
 
@@ -909,11 +906,6 @@ pub(crate) async fn welcome_message(context: &Context) -> String {
     translated(context, StockMessage::WelcomeMessage).await
 }
 
-/// Stock string: `Unknown sender for this chat.`.
-pub(crate) async fn unknown_sender_for_chat(context: &Context) -> String {
-    translated(context, StockMessage::UnknownSenderForChat).await
-}
-
 /// Stock string: `Message from %1$s`.
 // TODO: This can compute `self_name` itself instead of asking the caller to do this.
 pub(crate) async fn subject_for_new_contact(context: &Context, self_name: &str) -> String {