Backend: remove unused variable

Created by: camdencheek

This variable is unused and causing my editor to complain. It's been this way at least since the initial open source commit, so I think it's safe to just delete. To check my assumption here, we currently have 36 rows in the production DB with empty, but non-null verification codes, but they were all created in December of 2017.

Test plan

Just deleting dead code. Checked prod DB to make sure this isn't a larger issue than it appears.