Remove duplicated allow(dead_code) attribute

Accidentally added in #1552

Author: tarcieri

crates/intrinsic-test/src/json_parser.rs

@@ -17,7 +17,6 @@ struct ReturnType {
 #[serde(untagged, deny_unknown_fields)]
 pub enum ArgPrep {
     Register {
-        #[allow(dead_code)]
         #[serde(rename = "register")]
         #[allow(dead_code)]
         reg: String,