feat: do not reveal sender's language in read receipts (#5802)

while adapting strings for the recent change about read receipts,
https://github.com/deltachat/deltachat-core-rust/pull/5712 , it turns
out in discussions eg. at
https://github.com/deltachat/deltachat-android/issues/3179 that
untranslated english for the read receipts seem to be sufficient or even
better:

- do not reveal the sender's language
- unexpected languages are confusing - even if you chat in english, you
may get Chinese read receipts
- many clients do not show the text anyways, iirc, eg. Outlook display
the read receipts in context, and Delta Chat of course as well
- afaik, we're leaving comparable `multipart/report` untranslated as
well (sync, but also webxdc updates are practically english only)
- less code, fewer translations needed :)

src/stock_str.rs

@@ -80,9 +80,6 @@ pub enum StockMessage {
     #[strum(props(fallback = "Fingerprints"))]
     FingerPrints = 30,
 
-    #[strum(props(fallback = "Return receipt"))]
-    ReadRcpt = 31,
-
     #[strum(props(fallback = "End-to-end encryption preferred"))]
     E2ePreferred = 34,
 
@@ -440,9 +437,6 @@ pub enum StockMessage {
         fallback = "Could not yet establish guaranteed end-to-end encryption, but you may already send a message."
     ))]
     SecurejoinWaitTimeout = 191,
-
-    #[strum(props(fallback = "This message is a receipt notification."))]
-    ReadRcptMailBody = 192,
 }
 
 impl StockMessage {
@@ -795,16 +789,6 @@ pub(crate) async fn finger_prints(context: &Context) -> String {
     translated(context, StockMessage::FingerPrints).await
 }
 
-/// Stock string: `Return receipt`.
-pub(crate) async fn read_rcpt(context: &Context) -> String {
-    translated(context, StockMessage::ReadRcpt).await
-}
-
-/// Stock string: `This message is a receipt notification.`.
-pub(crate) async fn read_rcpt_mail_body(context: &Context) -> String {
-    translated(context, StockMessage::ReadRcptMailBody).await
-}
-
 /// Stock string: `Group image deleted.`.
 pub(crate) async fn msg_grp_img_deleted(context: &Context, by_contact: ContactId) -> String {
     if by_contact == ContactId::SELF {