Remove B type param (#1751)

Co-authored-by: Jonas Platte <[email protected]>
Co-authored-by: Michael Scofield <[email protected]>

axum-macros/tests/debug_handler/fail/duplicate_args.rs

@@ -1,9 +1,6 @@
 use axum_macros::debug_handler;
 
-#[debug_handler(body = BoxBody, body = BoxBody)]
+#[debug_handler(state = (), state = ())]
 async fn handler() {}
 
-#[debug_handler(state = (), state = ())]
-async fn handler_2() {}
-
 fn main() {}