simplify AA::Devise.config

Devise has `sign_out_via` since v1.2.0 (ab7f3bc1757104c053d3e3c9bf6db2de03dee830), and we have a min (soft) dependency to v3.2.x https://github.com/activeadmin/activeadmin/blob/master/lib/active_admin/dependency.rb#L3
Merge pull request #3607 from activeadmin/simplify_devise_config

simplify AA::Devise.config