ffmpeg: remove access to private FILE struct members on Windows

The FILE struct is opaque in MSVC 2015, and the members of this struct
were never meant to be accessed in any case.

No conditions are known where this check was needed to get characters
from stdin.

ffmpeg.c

@@ -434,10 +434,6 @@ static int read_key(void)
         is_pipe = !GetConsoleMode(input_handle, &dw);
     }
 
-    if (stdin->_cnt > 0) {
-        read(0, &ch, 1);
-        return ch;
-    }
     if (is_pipe) {
         /* When running under a GUI, you will end here. */
         if (!PeekNamedPipe(input_handle, NULL, 0, NULL, &nchars, NULL)) {