Remove obsolete reference to winflex download

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

doc/src/sgml/install-windows.sgml

@@ -255,15 +255,6 @@ $ENV{MSBFLAGS}="/m";
        </para>
      </note>
 
-     <note>
-       <para>
-       The obsolete <literal>winflex</literal> binaries distributed in the
-       downloads section of the PostgreSQL web site
-       and referenced in older documentation will fail with <quote>flex: fatal
-       internal error, exec failed</quote> on 64-bit Windows hosts. Use Flex from
-       MSYS instead.
-       </para>
-     </note>
      </listitem>
     </varlistentry>