Unused in this repo as of the earlier #6450 (300aba61a6)
and unused in the Windows GUI as of tailscale/corp#8065.
With this ipn.BackendServer is no longer used and could also be
removed from this repo. The macOS and iOS clients still temporarily
depend on it, but I can move it to that repo instead while and let its
migration proceed on its own schedule while we clean this repo up.
Updates #6417
Updates tailscale/corp#8051
Change-Id: Ie13f82af3eb9f96b3a21c56cdda51be31ddebdcf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
7e016c1d90