Use ..Default::default() to initialize TxIn

Clippy emits:

 field assignment outside of initializer for an instance created with
 Default::default()

As suggested, use `..Default` to initialize the `TxIn`. While we are at
it remove unnecessary comment (looks like it originated as commented out
code).

Author: tcharding

examples/psbt_sign_finalize.rs

@@ -80,10 +80,11 @@ fn main() {
 
     let (outpoint, witness_utxo) = get_vout(&depo_tx, &bridge_descriptor.script_pubkey());
 
-    let mut txin = TxIn::default();
-    txin.previous_output = outpoint;
-
-    txin.sequence = Sequence::from_height(26); //Sequence::MAX; //
+    let txin = TxIn {
+        previous_output: outpoint,
+        sequence: Sequence::from_height(26),
+        ..Default::default()
+    };
     psbt.unsigned_tx.input.push(txin);
 
     psbt.unsigned_tx