Remove call to user_params for new action in user controller

Calling `user_from_params` here takes away the possibility for users that
override the controller to override the `user_params` method with
`params.require(:user)`, which is a very common pattern for rails 4 and up
applications.

This also opens the door to remove the `Hash.new` call in the default
`user_params`, but I didn't want to go that far before understanding if
there is a good reason not to do this.