Remove unused admin email helper.

app/helpers/admin_email_helper.rb

-module AdminEmailHelper
-  def admin_email_grouped_recipient_options
-    options_for_select([['All GitLab users', 'all']]) +
-    grouped_options_for_select(
-      'Groups' => Group.pluck(:name, :id).map{ |name, id| [name, "group-#{id}"] },
-      'Projects' => grouped_project_list
-    )
-  end
-
-  protected
-  def grouped_project_list
-    Group.includes(:projects).flat_map do |group|
-      group.human_name
-      group.projects.map do |project|
-        ["#{group.human_name} / #{project.name}", "project-#{project.id}"]
-      end
-    end
-  end
-end