Cleanup old (non-existant) hosts

[TaiSHiNet]

If no mined data exists for a host, it should be deleted from the hostsfiles to keep it clean

[sf-devops]

This would be very useful!

[bentwire]

The only way I could think of actually doing this would be to use a reactor. There is no way to get the missing info on what needs to be deleted otherwise.

[simonclausen]

Just mark entries by the formula, so it can find them again, compare to mine and remove if not present.