avformat/avio: remove must_flush from AVIOContext

It is unused.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/avio.h

@@ -236,7 +236,6 @@ typedef struct AVIOContext {
     int (*write_packet)(void *opaque, uint8_t *buf, int buf_size);
     int64_t (*seek)(void *opaque, int64_t offset, int whence);
     int64_t pos;            /**< position in the file of the current buffer */
-    int must_flush;         /**< unused */
     int eof_reached;        /**< true if eof reached */
     int write_flag;         /**< true if open for writing */
     int max_packet_size;