Remove unnecessary explicit reference

Clippy emits various warnings of type:

  warning: this expression creates a reference which is immediately
  dereferenced by the compiler

As suggested, remove the unnecessary explicit reference.

Author: tcharding

src/util/amount.rs

@@ -2033,10 +2033,10 @@ mod tests {
 
         let json = "{\"amt\": 21000000.00000001, \
                     \"samt\": -21000000.00000001}";
-        let t: T = serde_json::from_str(&json).unwrap();
+        let t: T = serde_json::from_str(json).unwrap();
         assert_eq!(t, orig);
 
-        let value: serde_json::Value = serde_json::from_str(&json).unwrap();
+        let value: serde_json::Value = serde_json::from_str(json).unwrap();
         assert_eq!(t, serde_json::from_value(value).unwrap());
 
         // errors

src/util/key.rs

@@ -638,7 +638,7 @@ mod tests {
         ];
 
         let s = Secp256k1::new();
-        let sk = PrivateKey::from_str(&KEY_WIF).unwrap();
+        let sk = PrivateKey::from_str(KEY_WIF).unwrap();
         let pk = PublicKey::from_private_key(&s, &sk);
         let pk_u = PublicKey {
             inner: pk.inner,

src/util/misc.rs

@@ -317,7 +317,7 @@ mod tests {
 
         let secp = secp256k1::Secp256k1::new();
         let message = "rust-bitcoin MessageSignature test";
-        let msg_hash = super::signed_msg_hash(&message);
+        let msg_hash = super::signed_msg_hash(message);
         let msg = secp256k1::Message::from(msg_hash);
 
 
@@ -357,7 +357,7 @@ mod tests {
 
         let secp = secp256k1::Secp256k1::new();
         let message = "a different message from what was signed";
-        let msg_hash = super::signed_msg_hash(&message);
+        let msg_hash = super::signed_msg_hash(message);
 
         // Signature of msg = "rust-bitcoin MessageSignature test"
         // Signed with pk "UuOGDsfLPr4HIMKQX0ipjJeRaj1geCq3yPUF2COP5ME="