remove `seen` set
remove `seen` set

Summary: Dead instructions are unique by construction. Apparently, this didn't use the be the case at some point in ancient Redex history where cfg edges were represented in a less straightforward way, but that time is long gone.

Reviewed By: beicy

Differential Revision: D33354411

fbshipit-source-id: 0465f1c965f6e6025447a983d40e4643d5b9813f

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex