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) {
-		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) {
-			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]);
-	}
+	NSAssert(giftCard.identifier, @"Failed to remove gift card. Gift card must have a valid identifier.");
 	
-	return task;
+	NSURLComponents *components = [self URLComponentsForCheckoutsAppendingPath:[NSString stringWithFormat:@"gift_cards/%@", giftCard.identifier]
+																 checkoutToken:checkout.token
+																	queryItems:nil];
+	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);
+	}];
 }
 
 - (void)updateCheckout:(BUYCheckout *)checkout withGiftCardDictionary:(NSDictionary *)giftCardDictionary addingGiftCard:(BOOL)addingGiftCard