Auto merge of #1699 - m-ou-se:panic-format, r=RalfJung

Remove unnecessary `format!()` in `panic!()`.

`panic!(format!(..))` will start giving a warning [soon](rust-lang/rust#81645).

src/bin/miri.rs

@@ -230,10 +230,10 @@ fn main() {
                             err => panic!("unknown error decoding -Zmiri-seed as hex: {:?}", err),
                         });
                     if seed_raw.len() > 8 {
-                        panic!(format!(
+                        panic!(
                             "-Zmiri-seed must be at most 8 bytes, was {}",
                             seed_raw.len()
-                        ));
+                        );
                     }
 
                     let mut bytes = [0; 8];