doc/platform: drop reference to ffmpeg.zeranoe.com

It was closed in September 2020.

Fix issue:
http://trac.ffmpeg.org/ticket/9734

doc/platform.texi

@@ -92,9 +92,6 @@ For information about compiling FFmpeg on OS/2 see
 
 @chapter Windows
 
-To get help and instructions for building FFmpeg under Windows, check out
-the FFmpeg Windows Help Forum at @url{http://ffmpeg.zeranoe.com/forum/}.
-
 @section Native Windows compilation using MinGW or MinGW-w64
 
 FFmpeg can be built to run natively on Windows using the MinGW-w64