Remove unneeded includes and extends

From Rails 4 on, including `ActiveModel::Model` brings in the necessary modules previously included/extend in `EmailSignup`.

It also sets `persisted?` to a default of `false`, which enables us to remove that method from being declared. Since we are relying on `ActiveModel::Validations` and calling `validates_presence_of`, there is also no need to reopen the `valid?` method.