Add and apply Rails/RedundantActiveRecordAllMethod

This rule was introduced in rubocop-rails 2.21.0.

app/controllers/admin/signature_sheets_controller.rb

@@ -1,6 +1,6 @@
 class Admin::SignatureSheetsController < Admin::BaseController
   def index
-    @signature_sheets = SignatureSheet.all.order(created_at: :desc)
+    @signature_sheets = SignatureSheet.order(created_at: :desc)
   end
 
   def new