remove deprecated MissingHelperError proxy.

The error was moved outside of the `ClassMethods` module.

Author: senny

actionpack/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Remove deprecated `AbstractController::Helpers::ClassMethods::MissingHelperError`
+    in favor of `AbstractController::Helpers::MissingHelperError`.
+
+    *Yves Senn*
+
 *   Fix `assert_template` not being able to assert that no files were rendered.
 
     *Guo Xiang Tan*

actionpack/lib/abstract_controller/helpers.rb

@@ -27,9 +27,6 @@ def initialize(error, path)
     end
 
     module ClassMethods
-      MissingHelperError = ActiveSupport::Deprecation::DeprecatedConstantProxy.new('AbstractController::Helpers::ClassMethods::MissingHelperError',
-                                                                                   'AbstractController::Helpers::MissingHelperError')
-
       # When a class is inherited, wrap its helper module in a new module.
       # This ensures that the parent class's module can be changed
       # independently of the child class's.

guides/source/4_2_release_notes.md

@@ -94,6 +94,11 @@ Action Pack
 
 Please refer to the [Changelog][action-pack] for detailed changes.
 
+### Removals
+
+*   Removed deprecated `AbstractController::Helpers::ClassMethods::MissingHelperError`
+    in favor of `AbstractController::Helpers::MissingHelperError`.
+
 ### Deprecations
 
 *   Deprecated support for setting the `:to` option of a router to a symbol or a