refactor: Remove error stock strings that are rarely used these days (#7327)

This removes the DC_STR_CANTDECRYPT_MSG_BODY and
DC_STR_CANT_DECRYPT_OUTGOING_MSGS stock strings.

See
https://github.com/deltachat/deltachat-android/pull/3956#issuecomment-3425349195
for reasoning.

src/receive_imf.rs

@@ -1166,7 +1166,8 @@ async fn decide_chat_assignment(
             .await?;
         let now = tools::time();
         let update_config = if last_time.saturating_add(24 * 60 * 60) <= now {
-            let mut msg = Message::new_text(stock_str::cant_decrypt_outgoing_msgs(context).await);
+            let txt = "⚠️ It seems you are using Delta Chat on multiple devices that cannot decrypt each other's outgoing messages. To fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions.";
+            let mut msg = Message::new_text(txt.to_string());
             chat::add_device_msg(context, None, Some(&mut msg))
                 .await
                 .log_err(context)