Remove optional_code_owners_sections FF

optional_code_owners_sections has been introduced
in https://gitlab.com/gitlab-org/gitlab/-/issues/232995

enabled by default:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51643

config/feature_flags/development/optional_code_owners_sections.yml

----
-name: optional_code_owners_sections
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51002
-rollout_issue_url:
-milestone: '13.8'
-type: development
-group: group::source code
-default_enabled: true

doc/user/project/code_owners.md

@@ -230,6 +230,7 @@ the rules for "Groups" and "Documentation" sections:
 #### Optional Code Owners Sections **(PREMIUM)**
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/232995) in GitLab Premium 13.8 behind a feature flag, enabled by default.
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53227) in GitLab 13.9.
 
 To make a certain section optional, add a code owners section prepended with the
 caret `^` character. Approvals from owners listed in the section are **not** required. For example:

ee/app/models/approval_wrapped_code_owner_rule.rb

@@ -22,8 +22,6 @@ class ApprovalWrappedCodeOwnerRule < ApprovalWrappedRule
   end
 
   def section_optional?
-    return false unless Feature.enabled?(:optional_code_owners_sections, project, default_enabled: true)
-
     Gitlab::CodeOwners.optional_section?(project, merge_request.target_branch, section)
   end
 end

ee/spec/models/approval_wrapped_code_owner_rule_spec.rb

@@ -10,13 +10,12 @@ RSpec.describe ApprovalWrappedCodeOwnerRule do
   subject { described_class.new(merge_request, rule) }
 
   describe '#approvals_required' do
-    where(:feature_enabled, :optional_sections_enabled, :optional_section, :approver_count, :expected_required_approvals) do
-      true  | true  | false | 0 | 0
-      true  | true  | false | 2 | 1
-      true  | false | true  | 2 | 1
-      true  | true  | true  | 2 | 0
-      false | true  | false | 2 | 0
-      false | true  | false | 0 | 0
+    where(:feature_enabled, :optional_section, :approver_count, :expected_required_approvals) do
+      true  | false | 0 | 0
+      true  | false | 2 | 1
+      true  | true  | 2 | 0
+      false | false | 2 | 0
+      false | false | 0 | 0
     end
 
     with_them do
@@ -33,8 +32,6 @@ RSpec.describe ApprovalWrappedCodeOwnerRule do
           allow(subject.project)
             .to receive(:code_owner_approval_required_available?).and_return(true)
           allow(Gitlab::CodeOwners).to receive(:optional_section?).and_return(optional_section)
-
-          stub_feature_flags(optional_code_owners_sections: optional_sections_enabled)
         end
 
         context "when the project doesn't require code owner approval on all MRs" do