movenc: Get rid of a hack for updating the dvc1 atom

Use the more generic approach with the delay_moov flag, instead of
having a update mechanism specific to this one single atom.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.h

@@ -129,7 +129,6 @@ typedef struct MOVTrack {
     unsigned    frag_info_capacity;
 
     struct {
-        int64_t struct_offset;
         int     first_packet_seq;
         int     first_packet_entry;
         int     packet_seq;