Refactor remove gift card to assert instead of return error.

Mobile Buy SDK/Mobile Buy SDK/Data/BUYClient.m

@@ -378,23 +378,17 @@ NSString *const BUYClientCustomerAccessToken = @"X-Shopify-Customer-Access-Token
 
 - (NSURLSessionDataTask *)removeGiftCard:(BUYGiftCard *)giftCard fromCheckout:(BUYCheckout *)checkout completion:(BUYDataCheckoutBlock)block
 {
-	NSURLSessionDataTask *task = nil;
-	if (giftCard.identifier) {
+	NSAssert(giftCard.identifier, @"Failed to remove gift card. Gift card must have a valid identifier.");
+	
 	NSURLComponents *components = [self URLComponentsForCheckoutsAppendingPath:[NSString stringWithFormat:@"gift_cards/%@", giftCard.identifier]
 																 checkoutToken:checkout.token
 																	queryItems:nil];
-		task = [self deleteRequestForURL:components.URL completionHandler:^(NSDictionary *json, NSURLResponse *response, NSError *error) {
+	return [self deleteRequestForURL:components.URL completionHandler:^(NSDictionary *json, NSURLResponse *response, NSError *error) {
 		if (!error) {
 			[self updateCheckout:checkout withGiftCardDictionary:json[@"gift_card"] addingGiftCard:NO];
 		}
 		block(checkout, error);
 	}];
-	}
-	else {
-		block(nil, [NSError errorWithDomain:kShopifyError code:BUYShopifyError_NoGiftCardSpecified userInfo:nil]);
-	}
-	
-	return task;
 }
 
 - (void)updateCheckout:(BUYCheckout *)checkout withGiftCardDictionary:(NSDictionary *)giftCardDictionary addingGiftCard:(BOOL)addingGiftCard