Remove unnecessary call to clone

Clippy emits:

warning: using `clone` on type `secp256k1::XOnlyPublicKey` which
implements the `Copy` trait

As suggested, remove call to `clone`.

src/util/schnorr.rs

@@ -109,7 +109,7 @@ impl TapTweak for UntweakedPublicKey {
     /// The tweaked key and its parity.
     fn tap_tweak<C: Verification>(self, secp: &Secp256k1<C>, merkle_root: Option<TapBranchHash>) -> (TweakedPublicKey, secp256k1::Parity) {
         let tweak_value = TapTweakHash::from_key_and_tweak(self, merkle_root).into_inner();
-        let mut output_key = self.clone();
+        let mut output_key = self;
         let parity = output_key.tweak_add_assign(secp, &tweak_value).expect("Tap tweak failed");
 
         debug_assert!(self.tweak_add_check(secp, &output_key, parity, tweak_value));