Don't require the purchase button to have a specific ID attribute #8834

[cklosowski]

Fixes #8834

Proposed Changes:

1. Removes the assignment of the purchase button to a variable based on the ID
2. Uses the instance of $(this) to address the purchase button

[cklosowski]

Steps to replicate are here: #8834 (comment)

[robincornett]

This is working, but noting that PayPal Commerce Pro removes all of the checkout button filters with:

remove_all_filters( 'edd_checkout_button_purchase' );

so the sample code in the issue does not work for PayPal Commerce Pro.