Fix bulk action delete when singular label has been changed

The `process_bulk_action` function for discounts relies on the checkbox's name to be `download`. In my theme I filtered the singular name to be `product` using the `edd_default_downloads_name` filter and so bulk deleting the discount codes no longer works.

Default checkbox
![default](https://f.cloud.github.com/assets/52581/981840/87fcbf28-0793-11e3-8ff8-5aa604f2f5a7.png)

Singular label changed, checkbox takes on the label, thus breaking bulk edit delete
![label changed](https://f.cloud.github.com/assets/52581/981839/87f47b92-0793-11e3-864c-8dc507af543c.png)

Is there a more efficient way to do this than what I've done?