We used to distribute a binary version of flex for windows on our
download site, but it hasn't been working for many years. The "old
documentation" referenced was also for versions that have been EOL for
many years. So, remove it.

Discussion: https://postgr.es/m/CABUevEwXLJpVpab62f7AFXNWQ5=U0kvErCLq4VEsikidLyzSQg@mail.gmail.com