Adds service to deleteFactsFromSource

[bomoko]

This PR introduces the same functionality as an earlier PR #2611

The rationale for its existence mirrors that of the earlier PR, but mutatis mutandis wrt facts.

Essentially, it allows us to better target facts for clearing, since multiple facts could potentially exist with the same name, same source, but on different services.

Think, for instance, a php-version collected by some process that runs on both the php-nginx service and a cli service, we want to be able to clear the facts from one of the services, without having it be cleared from both.

General Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for inclusion in changelog

[shreddedbacon]

Good to merge from my perspective, this just extends an existing mutations input without any breaking changes, and the logic contained within appears sound for the description provided.