freezer-agent: '--remove-from-date' with '--always-level' doesn't remove certain backups

The agent doesn't always remove backups that it could safely remove, when
'--always-level' is enabled.

Here's the scenario:
  - 7 backups aged 60, 55, 50, 40, 15, 10, and 5 days before the current
    date are created (in this order)
  - '--always-level 3' is specified for each backup
  - Backups are then removed using '--remove-from-date' with the date
    30 days before the current date

My expectation is that the backup 40 days old should be deleted, since:
  - it falls before the minimum backup date, and
  - it will be a level 3 backup (i.e. max level), so it cannot have any
    dependent backups

However, none of the generated are deleted (40 included).

It's worth noting that backup removal in the trivial case *does* work. If
*all* backups are created before the `--remove-from-date` value, they'll
all be deleted. It's only when *childless* *sibling* backups fall after
the removal date that things are handled incorrectly.