Add voucher table cleanup when an llms_voucher post is deleted

[bsetiawan88]

Description

The voucher relationships in lifterlms_vouchers_codes are cleaned up when vouchers are deleted.

Fixes #1087

How has this been tested?

Manually.

Types of changes

Bug fix

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.

[actuallyakash]

Hey @bsetiawan88

Thanks for the contribution.

I'm gonna add:

• some docblocks
• a changelog file

[brianhogg]

@bsetiawan88 I don't seem to be able to push changes up to this fork, so I'll close this one and I've created a new one with props to you. Thanks again! https://github.com/gocodebox/lifterlms/pull/2652