We soft-delete AUMs when they're purged, but when we call `ChildAUMs()`,
we look up soft-deleted AUMs to find the `Children` field.
This patch changes the behaviour of `ChildAUMs()` so it only looks at
not-deleted AUMs. This means we don't need to record child information
on AUMs any more, which is a minor space saving for any newly-recorded
AUMs.
Updates https://github.com/tailscale/tailscale/issues/17566
Updates https://github.com/tailscale/corp/issues/27166
Signed-off-by: Alex Chan <alexc@tailscale.com>