flac muxer: accept only STREAMINFO extradata

The other format (full flac header blocks) should not be exported by any
demuxers anymore.

This allows to drop an avpriv_ function and also simplify the following
commits.

libavformat/flacenc_header.c

@@ -22,7 +22,7 @@
 #include "libavutil/channel_layout.h"
 
 #include "libavcodec/flac.h"
-#include "libavcodec/bytestream.h"
+
 #include "avformat.h"
 #include "flacenc.h"
 
@@ -32,18 +32,17 @@ int ff_flac_write_header(AVIOContext *pb, AVCodecContext *codec,
     uint8_t header[8] = {
         0x66, 0x4C, 0x61, 0x43, 0x00, 0x00, 0x00, 0x22
     };
-    uint8_t *streaminfo;
-    enum FLACExtradataFormat format;
 
     header[4] = last_block ? 0x80 : 0x00;
-    if (!avpriv_flac_is_extradata_valid(codec, &format, &streaminfo))
-        return -1;
+
+    if (codec->extradata_size < FLAC_STREAMINFO_SIZE)
+        return AVERROR_INVALIDDATA;
 
     /* write "fLaC" stream marker and first metadata block header */
     avio_write(pb, header, 8);
 
     /* write STREAMINFO */
-    avio_write(pb, streaminfo, FLAC_STREAMINFO_SIZE);
+    avio_write(pb, codec->extradata, FLAC_STREAMINFO_SIZE);
 
     return 0;
 }