This quest has NOT been removed from the game, but made unavailable to characters above leve 70! Simply speak to Skar'this the Heretic again to continue.
The NPC that gives this quest is located in heroic Slave Pens. Just after the first boss, after dropping down into the pool, he is in a cage on the left side of the next room. Originally, this was the first part of a quest line to gain attunement to Serpentshrine Cavern, though said attunement has been removed.
Burn it down, <race>. Burn it all down! They deserve nothing! Do you hear me! NOTHING! They must pay for what they have done. For what they continue to do.
I would kill Vashj myself if I could break free of these damned bindings.
Or maybe you seek to do the work of Neptulon? I will grant you the mark required to enter the Serpentshrine Cavern, but first you must bring me the artifacts I need to form the sacred cudgel.
Do you agree?