Remove local var devise_mapping; use the overridden `#devise_mappin…

…g` instead.

Instead of re-inventing `devise_mapping`, just use the overridden method that is
inherited from `DeviseController` in the `Devise` gem. The override takes the
`omniauth_params` into account, if there are any.

app/controllers/devise_token_auth/omniauth_callbacks_controller.rb

@@ -11,9 +11,7 @@ def redirect_callbacks
 
       # derive target redirect route from 'resource_class' param, which was set
       # before authentication.
-      devise_mapping = [request.env['omniauth.params']['namespace_name'],
-                        request.env['omniauth.params']['resource_class'].underscore.gsub('/', '_')].compact.join('_')
-      path = "#{Devise.mappings[devise_mapping.to_sym].fullpath}/#{params[:provider]}/callback"
+      path = "#{devise_mapping.fullpath}/#{params[:provider]}/callback"
       klass = request.scheme == 'https' ? URI::HTTPS : URI::HTTP
       redirect_route = klass.build(host: request.host, port: request.port, path: path).to_s