Now that we support using an in-memory backend for TKA state (#17946),
this function always returns `nil` – we can always support Network Lock.
We don't need it any more.
Plus, clean up a couple of errant TODOs from that PR.
Updates tailscale/corp#33599
Change-Id: Ief93bb9adebb82b9ad1b3e406d1ae9d2fa234877
Signed-off-by: Alex Chan <alexc@tailscale.com>