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/receive_imf_tests.rs

@@ -3271,7 +3271,7 @@ async fn test_outgoing_undecryptable() -> Result<()> {
     assert!(
         dev_msg
             .text
-            .contains(&stock_str::cant_decrypt_outgoing_msgs(alice).await)
+            .contains("⚠️ 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 raw = include_bytes!("../../test-data/message/thunderbird_encrypted_signed.eml");