Coinciding with Rigsy's reappearance in Who, it turns out that Ashildr has been using a drug on him in order to wipe his mind of the memory of where the trap street is located. She uses an amnesia pill for this, otherwise known as Retcon. The drug was already establish in the Whoniverse long before now, having made its debut appearance in the first episode of spin-off Torchwood, Everything Changes, with Captain Jack giving it to Gwen. It appeared frequently throughout the show, most recently in Miracle Day. Since it doesn't seem likely we'll be getting more Torchwood anytime soon, it's a logical step to use the drug here, which at least ensures it keeps a place in the universe.