Remove backend use of manual_sorting feature flag

app/controllers/groups_controller.rb

@@ -7,10 +7,6 @@ class GroupsController < Groups::ApplicationController
   include PreviewMarkdown
   include RecordUserLastActivity
 
-  before_action do
-    push_frontend_feature_flag(:manual_sorting, default_enabled: true)
-  end
-
   respond_to :html
 
   prepend_before_action(only: [:show, :issues]) { authenticate_sessionless_user!(:rss) }

app/controllers/projects/issues_controller.rb

@@ -10,10 +10,6 @@ class Projects::IssuesController < Projects::ApplicationController
   include SpammableActions
   include RecordUserLastActivity
 
-  before_action do
-    push_frontend_feature_flag(:manual_sorting, default_enabled: true)
-  end
-
   def issue_except_actions
     %i[index calendar new create bulk_update import_csv]
   end

app/views/shared/issuable/_sort_dropdown.html.haml

 - sort_value = @sort
 - sort_title = issuable_sort_option_title(sort_value)
 - viewing_issues = controller.controller_name == 'issues' || controller.action_name == 'issues'
-- manual_sorting = viewing_issues && controller.controller_name != 'dashboard' && Feature.enabled?(:manual_sorting, default_enabled: true)
+- manual_sorting = viewing_issues && controller.controller_name != 'dashboard'
 
 .dropdown.inline.prepend-left-10.issue-sort-dropdown
   .btn-group{ role: 'group' }