movenc: Don't check the custom IO flag when using faststart

The custom IO flag actually never is set for muxers, only for
demuxers, so the check was pointless (unless a user intentionally
would set the flag to signal using custom IO).
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -3437,10 +3437,9 @@ static int mov_write_header(AVFormatContext *s)
 
     /* faststart: moov at the beginning of the file, if supported */
     if (mov->flags & FF_MOV_FLAG_FASTSTART) {
-        if ((mov->flags & FF_MOV_FLAG_FRAGMENT) ||
-            (s->flags & AVFMT_FLAG_CUSTOM_IO)) {
+        if (mov->flags & FF_MOV_FLAG_FRAGMENT) {
             av_log(s, AV_LOG_WARNING, "The faststart flag is incompatible "
-                   "with fragmentation and custom IO, disabling faststart\n");
+                   "with fragmentation, disabling faststart\n");
             mov->flags &= ~FF_MOV_FLAG_FASTSTART;
         }
     }