Remove any existing subscriptions before adding a new save metaboxes sub to prevent multiple saves

[glendaviesnz]

Description

Currently multiple calls to setAvailableMetaBoxesPerLocation() action causes multiple AJAX requests to post.php to be made as each call initiates a new store subscription. This change removes any existing subscription before adding a new one to ensure only one sub is in play at a time.

How has this been tested?

1. Create a new post
2. Open Options and enable the Custom Fields advanced panel
3. Type some content in the post
4. Open DevTools and run this code in the Console tab:
   wp.data.dispatch( 'core/edit-post' ).setAvailableMetaBoxesPerLocation( {"side":[],"normal":[{"id":"postcustom","title":"Custom Fields"}],"advanced":[]} );
5. Press Publish
6. Open DevTools and look in the Network tab and make sure there is only one requests to post.php with each publish/update

Types of changes

Fixes #14759

Checklist:

• [ X ] Only manually tested - there are no existing unit tests for the effects, and it would require a considerable amount of mocking to add any useful tests
• [ X ] My code follows the WordPress code style.
• [ NA ] My code follows the accessibility standards.
• [ X ] My code has proper inline documentation.
• [ NA ] I've included developer documentation if appropriate.

[glendaviesnz]

Have just left as draft for now as want to a bit more testing of it tomorrow.

[elliotcondon]

@glendaviesnz Great work! 🥳🎉🎉🎉

You have just made a lot of ACF users very happy indeed.
Is this likely to make its way into WP 5.3 or a later version?

[glendaviesnz]

You have just made a lot of ACF users very happy indeed.
Is this likely to make its way into WP 5.3 or a later version?

no worries, @elliotcondon, turned out to be a pretty easy fix. I am pretty new to contributing sorry, so not sure how the release cycle is working for WP 5.3 - @youknowriad do you know the answer to that?

[youknowriad]

yes, this will be included in WordPress 5.3, you can check with beta2, it's already there.