Remove Signature's argument_bytes field.

It's not currently used. If we do need such information, it would be
better to compute it on demand.

lib/reader/src/parser.rs

@@ -919,10 +919,6 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if sig.params.iter().all(|a| a.location.is_assigned()) {
-            sig.compute_argument_bytes();
-        }
-
         Ok(sig)
     }