Trader minor cleanups (coinbase#172)

* Trader#cancelOrder: remove then block that only returns its input.

* Trader#handleCancelOrder: remove unclear return keyword.

Don't return the result of this.emitMessageAsync() because it doesn't
return anything.

* Remove unnecessary uses of Promise.resolve().

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then#Return_value

src/core/Trader.ts

@@ -110,15 +110,14 @@ export class Trader extends Writable {
             // We pass the message along, but let the user decide what to do
             // We also have to wrap this call in a setImmediate; else any errors in the event handler will get thrown from here and lead to an unhandledRejection
             this.emitMessageAsync('Trader.place-order-failed', err.asMessage());
-            return Promise.resolve(null);
+            return null;
         });
     }
 
     cancelOrder(orderId: string): Promise<string> {
-        return this.api.cancelOrder(orderId).then((id: string) => {
-            // To avoid race conditions, we only actually remove the order when the tradeFinalized message arrives
-            return id;
-        });
+        // To avoid race conditions, we only actually remove the order
+        // when the tradeFinalized message arrives.
+        return this.api.cancelOrder(orderId);
     }
 
     cancelMyOrders(): Promise<string[]> {
@@ -165,8 +164,7 @@ export class Trader extends Writable {
             actualOrders.forEach((order: LiveOrder) => {
                 book.add(order);
             });
-            const diff = OrderbookDiff.compareByOrder(this.myBook, book);
-            return Promise.resolve(diff);
+            return OrderbookDiff.compareByOrder(this.myBook, book);
         });
     }
 
@@ -217,7 +215,7 @@ export class Trader extends Writable {
 
     private handleCancelOrder(request: CancelOrderRequestMessage) {
         this.cancelOrder(request.orderId).then((result: string) => {
-            return this.emitMessageAsync('Trader.order-cancelled', result);
+            this.emitMessageAsync('Trader.order-cancelled', result);
         }, (err: Error) => {
             this.emitMessageAsync('Trader.cancel-order-failed', err);
         });