Remove frozen_string_literal comment from activestorage's migration

The activestorage's migration is used as template for apps
Related to #30348

.rubocop.yml

@@ -88,6 +88,7 @@ Style/FrozenStringLiteralComment:
     - 'actionview/test/**/*.ruby'
     - 'actionpack/test/**/*.builder'
     - 'actionpack/test/**/*.ruby'
+    - 'activestorage/db/migrate/**/*.rb'
 
 # Use `foo {}` not `foo{}`.
 Layout/SpaceBeforeBlockBraces:

activestorage/db/migrate/20170806125915_create_active_storage_tables.rb

@@ -1,5 +1,3 @@
-# frozen_string_literal: true
-
 class CreateActiveStorageTables < ActiveRecord::Migration[5.1]
   def change
     create_table :active_storage_blobs do |t|