The `recovery_mode_clean_expired_keys` cron event is orphaned after converting to Multisite

[johnbillion]

To reproduce:

1. Create a single site installation
2. Convert it to a Multisite installation, for example via wp core multisite-convert or via the Tools -> Network Setup menu
3. Observe that the recovery_mode_clean_expired_keys cron event is still scheduled to run daily but that it does not have a registered callback and is therefore useless

To view the callbacks for a cron event hook you can use the WP Crontrol plugin or run some code such as has_action('recovery_mode_clean_expired_keys').

This cron event is orphaned because recovery mode is not operational on a Multisite installation, therefore the WP_Recovery_Mode class isn't instantiated.

Ideally the recovery_mode_clean_expired_keys cron event would be deleted upon installation of Multisite.

Trac ticket: https://core.trac.wordpress.org/ticket/61450

[narenin]

Hi @johnbillion

I have submitted the PR with patch, could you please take a look.

​https://github.com/WordPress/wordpress-develop/pull/6835