Issue #1437798 by stewart.adam: remove (out of scope) stock hook.
Also fixes an issue where uc_ajax_stock violated the uc_stock namespace. Note
that this change requires users use another module if they would like to
continue preventing out of stock products from being added to cart.
Issue #1455636 by stewart.adam: No discounts could cause a PHP error message
Fixes a problem where uc_ajax_cart_preprocess_uc_ajax_cart_block_content()
could end prematurely if no discounts were in place, causing $items to be
stdClass objects instead of arrays in the theme template.