Add class name to "remove" button in cart template.

The remove button in the default cart template doesn't have a class name. This makes it awkward to add a css style to it while developing themes. I have added a class name of "edd-cart-remove-item-btn" so that styles may easily be applied.
Hyphens to Underscores on Class Name
Merge pull request #2321 from mintplugins/master

Add class name to "remove" button in cart template.