chore(backups/_VmBackup#_copyDelta): remove useless check
chore(backups/_VmBackup#_copyDelta): remove useless check

All delta writers now have a `prepare()` method since e0d6b501c

lmcro/xo-web,lmcro/xo-web,vatesfr/xo-web,lmcro/xo-web,vatesfr/xo-web