[Tagging] Sourced from somewhere it shouldn't have been

[Dave F.]

Hi

I've a railway that has source as from 7th series maps, which aren't out 
of copyright yet.
I think the user meant NPE (when was it introduced into OSM?)

I can remove the tag from the current way but how do I remove from all 
the historic items?
I remember this problem being discussed recently but I'm damned if I can 
find it.

Cheers
Dave F.