net/netcheck: remove unused method from interface

Updates #2331
Updates #5162

Change-Id: I77ff956c2d59bde611d47523659a07afb4a6da2d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -208,7 +208,6 @@ type Client struct {
 // reusing an existing UDP connection.
 type STUNConn interface {
 	WriteToUDPAddrPort([]byte, netip.AddrPort) (int, error)
-	WriteTo([]byte, net.Addr) (int, error)
 	ReadFrom([]byte) (int, net.Addr, error)
 }