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/stock_str.rs

@@ -71,9 +71,6 @@ pub enum StockMessage {
     #[strum(props(fallback = "No encryption"))]
     EncrNone = 28,
 
-    #[strum(props(fallback = "This message was encrypted for another setup."))]
-    CantDecryptMsgBody = 29,
-
     #[strum(props(fallback = "Fingerprints"))]
     FingerPrints = 30,
 
@@ -392,11 +389,6 @@ pub enum StockMessage {
     ))]
     InvalidUnencryptedMail = 174,
 
-    #[strum(props(
-        fallback = "⚠️ 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."
-    ))]
-    CantDecryptOutgoingMsgs = 175,
-
     #[strum(props(fallback = "You reacted %1$s to \"%2$s\""))]
     MsgYouReacted = 176,
 
@@ -763,16 +755,6 @@ pub(crate) async fn encr_none(context: &Context) -> String {
     translated(context, StockMessage::EncrNone).await
 }
 
-/// Stock string: `This message was encrypted for another setup.`.
-pub(crate) async fn cant_decrypt_msg_body(context: &Context) -> String {
-    translated(context, StockMessage::CantDecryptMsgBody).await
-}
-
-/// Stock string:`Got outgoing message(s) encrypted for another setup...`.
-pub(crate) async fn cant_decrypt_outgoing_msgs(context: &Context) -> String {
-    translated(context, StockMessage::CantDecryptOutgoingMsgs).await
-}
-
 /// Stock string: `Fingerprints`.
 pub(crate) async fn finger_prints(context: &Context) -> String {
     translated(context, StockMessage::FingerPrints).await