Remove a deprecated Devise setting

This generated a deprecation warning, and was ignored by Devise.
It was also always irrelevant to Signonotron2 (it was for schemaless
databases - eg mongo, not mysql - which we are not using).
Remove deprecated Devise setting

This setting has had no effect since Devise 2.0, and was generating a
deprecation warning:
https://github.com/plataformatec/devise/wiki/How-To:-Upgrade-to-Devise-2.1