Remove migration base class switcher from `RailsSettingsMigration` (#29047)

db/migrate/20161006213403_rails_settings_migration.rb

@@ -1,12 +1,6 @@
 # frozen_string_literal: true
 
-MIGRATION_BASE_CLASS = if ActiveRecord::VERSION::MAJOR >= 5
-                         ActiveRecord::Migration[5.0]
-                       else
-                         ActiveRecord::Migration[4.2]
-                       end
-
-class RailsSettingsMigration < MIGRATION_BASE_CLASS
+class RailsSettingsMigration < ActiveRecord::Migration[5.0]
   def self.up
     create_table :settings do |t|
       t.string     :var, null: false