refactor: remove unnecessary allow(clippy::indexing_slicing)

clippy::indexing_slicing is already allowed in test builds.

src/mimefactory.rs

@@ -2274,7 +2274,6 @@ mod tests {
         let msg = message.as_string();
 
         let header_end = msg.find("Hi").unwrap();
-        #[allow(clippy::indexing_slicing)]
         let headers = msg[0..header_end].trim();
 
         assert!(!headers.lines().any(|l| l.trim().is_empty()));