Make "Remove" from Cart Button work within Ajax calls

In the "edd_remove_item_url" function, there is a Boolean parameter for $ajax - however, it isn't actually used - neither is it actually necessary.

I have modified the function so it can be used in ajax functions and uses the checkout page by default.

The change involves checking the "DOING_AJAX" constant, and if we are using ajax, we use the edd_get_checkout_uri() function as the $current_page used to create the URL in the "Remove" button.