coins: remove useless undo method.

lib/coins/undocoins.js

@@ -32,10 +32,11 @@ function UndoCoins() {
 /**
  * Push coin entry onto undo coin array.
  * @param {CoinEntry}
+ * @returns {Number}
  */
 
 UndoCoins.prototype.push = function push(coin) {
-  this.items.push(coin);
+  return this.items.push(coin);
 };
 
 /**
@@ -118,15 +119,6 @@ UndoCoins.prototype.commit = function commit() {
   return raw;
 };
 
-/**
- * Retrieve the last undo coin.
- * @returns {UndoCoin}
- */
-
-UndoCoins.prototype.top = function top() {
-  return this.items[this.items.length - 1];
-};
-
 /**
  * Re-apply undo coins to a view, effectively unspending them.
  * @param {CoinView} view