fix(test-support): remove special case for $message_type

crates/cargo-test-support/src/compare.rs

@@ -596,18 +596,6 @@ fn find_json_mismatch_r<'a>(
             let mut expected_entries = l.iter();
             let mut actual_entries = r.iter();
 
-            // Compilers older than 1.76 do not produce $message_type.
-            // Treat it as optional for now.
-            let mut expected_entries_without_message_type;
-            let expected_entries: &mut dyn Iterator<Item = _> =
-                if l.contains_key("$message_type") && !r.contains_key("$message_type") {
-                    expected_entries_without_message_type =
-                        expected_entries.filter(|entry| entry.0 != "$message_type");
-                    &mut expected_entries_without_message_type
-                } else {
-                    &mut expected_entries
-                };
-
             loop {
                 match (expected_entries.next(), actual_entries.next()) {
                     (None, None) => return None,