RG can remove with defense up in Internet challenge but cannot in raids.
The debuff is called the same in both raids and challenges, but seems to work a little but differently with same effect almost instant death.
The captian ability in raids that applies deathmark is called kill order
Mace ability in challenge that applies death marked is called shatterpoint.
Speculation...
I don't believe there are any tanks that have debuff removal so it was designed that buffing you team would remove the debuff in the challenges. Raids can have characters that can remove debuffs.