refactor(proto): Remove unused protocol flags and helper functions

crates/irtt-proto/src/echo.rs

@@ -1,5 +1,5 @@
 use crate::{
-    flags::{has, FLAG_CLOSE, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY},
+    flags::{has, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY},
     hmac,
     layout::{echo_packet_len, PacketLayout},
     open::{check_hmac_presence, reject},
@@ -147,11 +147,6 @@ fn read_i64(packet: &[u8], pos: &mut usize) -> i64 {
     value
 }
 
-#[allow(dead_code)]
-fn _assert_close_flag_imported(_: u8) {
-    let _ = FLAG_CLOSE;
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;