Revert "In Rails >= 5, use optional: true, not required: false (#6596)"

This reverts commit a5105d8.

Author: bquorning

lib/rubocop/cop/rails/belongs_to.rb

@@ -14,7 +14,6 @@ module Cop
 require_relative 'rails/application_job'
 require_relative 'rails/application_record'
 require_relative 'rails/assert_not'
-require_relative 'rails/belongs_to'
 require_relative 'rails/blank'
 require_relative 'rails/bulk_change_table'
 require_relative 'rails/create_table_with_timestamps'

lib/rubocop/cop/rails_cops.rb

@@ -162,49 +162,6 @@ Name | Default value | Configurable values
 --- | --- | ---
 Include | `**/test/**/*` | Array
 
-## Rails/BelongsTo
-
-Enabled by default | Supports autocorrection
---- | ---
-Enabled | Yes
-
-This cop looks for belongs_to associations where we control whether the
-association is required via the deprecated `required` option instead.
-
-Since Rails 5, belongs_to associations are required by default and this
-can be controlled through the use of `optional: true`.
-
-From the release notes:
-
-    belongs_to will now trigger a validation error by default if the
-    association is not present. You can turn this off on a
-    per-association basis with optional: true. Also deprecate required
-    option in favor of optional for belongs_to. (Pull Request)
-
-In the case that the developer is doing `required: false`, we
-definitely want to autocorrect to `optional: true`.
-
-However, without knowing whether they've set overriden the default
-value of `config.active_record.belongs_to_required_by_default`, we
-can't say whether it's safe to remove `required: true` or replace it
-with `optional: false` (or, similarly, remove a superfluous `optional:
-false`). Therefore, in the cases we're using `required: true`, we'll
-highlight that `required` is deprecated but otherwise do nothing.
-
-### Examples
-
-```ruby
-# bad
-class Post < ApplicationRecord
-  belongs_to :blog, required: false
-end
-
-# good
-class Post < ApplicationRecord
-  belongs_to :blog, optional: true
-end
-```
-
 ## Rails/Blank
 
 Enabled by default | Supports autocorrection