Just use the module function

app/helpers/license_helper.rb

@@ -80,7 +80,7 @@ module LicenseHelper
     return @show_promotions if defined?(@show_promotions)
 
     @show_promotions =
-      if current_application_settings.should_check_namespace_plan?
+      if Gitlab::CurrentSettings.current_application_settings.should_check_namespace_plan?
         true
       else
         license = License.current

app/models/issue_assignee.rb

 class IssueAssignee < ActiveRecord::Base
-  extend Gitlab::CurrentSettings
-
   belongs_to :issue
   belongs_to :assignee, class_name: "User", foreign_key: :user_id
 
@@ -9,7 +7,7 @@ class IssueAssignee < ActiveRecord::Base
   # EE-specific
 
   def update_elasticsearch_index
-    if current_application_settings.elasticsearch_indexing?
+    if Gitlab::CurrentSettings.current_application_settings.elasticsearch_indexing?
       ElasticIndexerWorker.perform_async(
         :update,
         'Issue',