Custom notifications on downed hosts also send another down notification

[dasJ]

Describe the bug

I have a host that is down and that has notifications with interval = 0 so renotifications are disabled. When the initial notification was sent and I send another custom notification, it sends another down notification.

To Reproduce

1. Have a host with notifications that have interval = 0
2. Have the host in a down state
3. Wait for the notification to arrive
4. Send a custom notification for this host

Expected behavior

I should only receive a custom notification and not also a down notification shortly afterwards.

Your Environment

Include as many relevant details about the environment you experienced the problem in

• Version used (icinga2 --version): r2.13.0-1
• Operating System and version: NixOS 21.05
• Enabled features (icinga2 feature list): api checker ido-mysql notification syslog
• Icinga Web 2 version and modules (System - About): Irrelevant
• Config validation (icinga2 daemon -C): It's valid
• If you run multiple Icinga 2 instances, the zones.conf file (or icinga2 object list --type Endpoint and icinga2 object list --type Zone) from all affected nodes. Irrelevant

Additional context

[Al2Klimov]

Sounds like #7818

[yhabteab]

Duplicate of #6333