Remove deprecated stuff (#5803)

All of these have been deprecated for years, if we're releasing a new
major version, let's take the opportunity to do some cleanup.

* Remove deprecated `:bypass` option from `sign_in` helper,
  use `bypass_sign_in` instead.
* Remove deprecated `devise_error_messages!` helper,
  use `render "devise/shared/error_messages", resource: resource` instead.
* Remove deprecated `scope` second argument from `sign_in(resource, :admin)`
  controller test helper, use `sign_in(resource, scope: :admin)` instead.
* Remove deprecated `Devise::TestHelpers`,
  use `Devise::Test::ControllerHelpers` instead.

Closes #5739

test/controllers/custom_strategy_test.rb

@@ -3,7 +3,7 @@
 require 'test_helper'
 require 'ostruct'
 require 'warden/strategies/base'
-require 'devise/test_helpers'
+require 'devise/test/controller_helpers'
 
 class CustomStrategyController < ActionController::Base
   def new