Update trash_users_subscriptions() to properly delete subscriptions in HPOS environments.

[mattallan]

Description

When a user/subscriber is deleted, we hook onto the delete_user hook and delete all subscriptions that belong to this user. This function was still using wp_post_delete() which doesn't work in a HPOS environment resulting in subscriptions remaining without a customer attached.

This PR updates the trash_users_subscriptions() function and replaces the wp_post_delete( $id ) method with $subscription->delete( true ) so work in both HPOS & WP Posts environments.

How to test this PR

1. Enable High-performance order storage on your store (found: WC > Settings > Advanced > Features)
2. Create a throw-away user by going to WP Admin > Users > Add New
3. Create the user with the minimum details i.e. customer1 [email protected] and default password
4. Go to WC > Subscriptions > Add New
5. Create a basic subscription for this new customer and use the actions drop-down to create a pending parent order.
6. Go to WP Admin > Users and delete the user.
7. On trunk you will notice the subscription still exists without a customer and the parent order will have a note saying the subscription was deleted.
8. On this branch the subscription will be removed.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes
• Will this PR affect WooCommerce Payments? yes

[james-allan]

Changes look good to me and work as described.

I replicated the issue on trunk - the customer subscriptions remained after I deleted the user.

I confirmed this also works while syncing is enabled (it deletes in both tables) with both HPOS and WP Posts tables.

• HPOS - no syncing
• HPOS - with syncing
• WP Posts - no syncing
• WP Posts - with syncing