all: cleanup unused code, part 1 (#10661)

Run `staticcheck` with `U1000` to find unused code. This cleans up about
a half of it. I'll do the other half separately to keep PRs manageable.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

wgengine/router/router_linux.go

@@ -605,10 +605,6 @@ func (r *linuxRouter) getV6Available() bool {
 	return r.nfr.HasIPV6()
 }
 
-func (r *linuxRouter) getV6NATAvailable() bool {
-	return r.nfr.HasIPV6NAT()
-}
-
 // addAddress adds an IP/mask to the tunnel interface. Fails if the
 // address is already assigned to the interface, or if the addition
 // fails.

wgengine/userspace.go

@@ -985,8 +985,6 @@ func (e *userspaceEngine) getStatusCallback() StatusCallback {
 	return e.statusCallback
 }
 
-var singleNewline = []byte{'\n'}
-
 var ErrEngineClosing = errors.New("engine closing; no status")
 
 func (e *userspaceEngine) getPeerStatusLite(pk key.NodePublic) (status ipnstate.PeerStatusLite, ok bool) {
@@ -1487,8 +1485,7 @@ func (ls fwdDNSLinkSelector) PickLink(ip netip.Addr) (linkName string) {
 }
 
 var (
-	metricMagicDNSPacketIn = clientmetric.NewCounter("magicdns_packet_in") // for 100.100.100.100
-	metricReflectToOS      = clientmetric.NewCounter("packet_reflect_to_os")
+	metricReflectToOS = clientmetric.NewCounter("packet_reflect_to_os")
 
 	metricNumMajorChanges = clientmetric.NewCounter("wgengine_major_changes")
 	metricNumMinorChanges = clientmetric.NewCounter("wgengine_minor_changes")