Fix issue with disabled mixed checkout

[Mayisha]

Fixes #130 #560

Description

This PR handles two issues related to the mixed checkout.

• When a simple product is added to cart with an existing subscription product and subscription rules avoid to have both types, Add to cart Ajax call gets a 500 response. The error is caused by trying to use non-existing function redirect_ajax_add_to_cart in WC_Subscriptions_Cart_Validator class.

PHP Fatal error:  Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, class WC_Subscriptions_Cart_Validator does not have a method "redirect_ajax_add_to_cart" in C:\xampp\htdocs\testsubscriptions\wp-includes\class-wp-hook.php:307

In WC_Subscriptions_Cart_Validator class we were using redirect_ajax_add_to_cart as the callback which does not exist. This function is replaced by add_to_cart_ajax_redirect, handled by the deprecation handler class. In https://github.com/Automattic/woocommerce-subscriptions-core/pull/606/commits/118ef402104c54fbef89f2b788294954ee416259, I am now directly using the add_to_cart_ajax_redirect as the callback. I could not reproduce this issue but using add_to_cart_ajax_redirect directly as the callback makes more sense here.

• When mixed checkout is disabled, and a subscription is in the cart when another product type is added, the customer is provided with a notice A subscription has been removed from your cart. Products and subscriptions can not be purchased at the same time. But when a subscription is added to the cart that already contains another product, no notice is provided. Addressed this in https://github.com/Automattic/woocommerce-subscriptions-core/pull/606/commits/152cb9d7a5bcd717e8c107c8d181a301a1196b72, 6fdf3be,now showing a notice on the page when the other products are removed from the cart because of adding a subscription.

How to test this PR

Issue #130

1. Disabled mixed checkout from WooCommerce > Settings > Subscriptions.
2. Create a variable subscription. On the product edit page's Advance section, limit the subscription to one active subscription at a time.
3. As a shopper, add the variable subscription to the cart.
4. Now add another simple product to the cart.
5. The simple product should be added to the cart without any error.

Issue #560

1. Disabled mixed checkout from WooCommerce > Settings > Subscriptions.
2. As a shopper, add a subscription to the cart.
3. Now add another simple product to the cart. You should see the following notice on the page.

5. Clear the cart and add a subscription again.
6. Now add a second subscription to the cart. You should see the following notice on the page.

7. Clear the cart and add one or a couple of simple products to the cart.
8. Now add a subscription to the cart. You should see the following notice on the page.