We strip them control-side anyway, and we already strip IPv4 link
local, so there's no point uploading them. And iOS has a ton of them,
which results in somewhat silly amount of traffic in the MapRequest.
We'll be doing same-LAN-inter-tailscaled link-local traffic a
different way, with same-LAN discovery.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>