box: Remove an unused macro isom_copy_fields.
box: Remove an unused macro isom_copy_fields.

dwbuiten/l-smash,silverfilain/L-SMASH,canbal/l-smash,mstorsjo/l-smash,silverfilain/L-SMASH,dwbuiten/l-smash,silverfilain/L-SMASH,maki-rxrz/L-SMASH,canbal/l-smash,l-smash/l-smash,l-smash/l-smash,l-smash/l-smash,mstorsjo/l-smash,maki-rxrz/L-SMASH