Winflex docs and distro

Since PostgreSQL 9.3, in commit a266f7dd93b, we've added the text:

+       The obsolete "winflex" binaries distributed on the PostgreSQL FTP site
+       and referenced in older documentation will fail with "flex: fatal
+       internal error, exec failed" on 64-bit Windows hosts. Use flex from
+       msys instead.

At this point. I suggest we simply stop distributing winflex on our
download site, and just remove this note from the documentation. (This
is just a note, the general documentation still says get flex from
msys, separately).

Surely a binary that doesn't work on a 64-bit system is not of help to
anybody these days.. And "older documentation" now refers to 9.2 which
was EOL in 2017.

-- 
 Magnus Hagander
 Me: https://www.hagander.net/
 Work: https://www.redpill-linpro.com/