The one remaining caller of peerMap.endpointForDiscoKey was making the
improper assumption that there's exactly 1 node with a given DiscoKey
in the network. That was the cause of #3088.
Now that all the other callers have been updated to not use
endpointForDiscoKey, there's no need to try to keep maintaining that
prone-to-misuse index.
Updates #3088
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 695df497ba)
f054e16451