nullenc: drop AVFMT_RAWPICTURE from the flags

This makes the two pass encoding with x264 working with -f null - as
first pass.

libavformat/nullenc.c

@@ -32,5 +32,5 @@ AVOutputFormat ff_null_muxer = {
     .audio_codec       = AV_NE(CODEC_ID_PCM_S16BE, CODEC_ID_PCM_S16LE),
     .video_codec       = CODEC_ID_RAWVIDEO,
     .write_packet      = null_write_packet,
-    .flags = AVFMT_NOFILE | AVFMT_RAWPICTURE | AVFMT_NOTIMESTAMPS,
+    .flags = AVFMT_NOFILE | AVFMT_NOTIMESTAMPS,
 };