refactor: remove unused allow(clippy::indexing_slicing) for heuristically_parse_ndn

src/mimeparser.rs

@@ -1753,7 +1753,6 @@ impl MimeMessage {
     /// Some providers like GMX and Yahoo do not send standard NDNs (Non Delivery notifications).
     /// If you improve heuristics here you might also have to change prefetch_should_download() in imap/mod.rs.
     /// Also you should add a test in receive_imf.rs (there already are lots of test_parse_ndn_* tests).
-    #[allow(clippy::indexing_slicing)]
     async fn heuristically_parse_ndn(&mut self, context: &Context) {
         let maybe_ndn = if let Some(from) = self.get_header(HeaderDef::From_) {
             let from = from.to_ascii_lowercase();