Remove `sse_erb_support` feature flag

Enable `.md.erb` support for everybody

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/235460
Feature flag MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39195

changelogs/unreleased/235460_remove_sse_erb_support_feature_flag.yml

+---
+title: Add support for .md.erb files in Static Site Editor
+merge_request: 42353
+author:
+type: added

config/feature_flags/development/sse_erb_support.yml

----
-name: sse_erb_support
-introduced_by_url:
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/235460
-group: group::static site editor
-type: development
-default_enabled: false

lib/gitlab/static_site_editor/config/generated_config.rb

@@ -34,7 +34,7 @@ module Gitlab
         delegate :project, to: :repository
 
         def supported_extensions
-          %w[.md].freeze
+          %w[.md .md.erb].freeze
         end
 
         def commit_id
@@ -50,8 +50,6 @@ module Gitlab
         end
 
         def extension_supported?
-          return true if path.end_with?('.md.erb') && Feature.enabled?(:sse_erb_support, project)
-
           supported_extensions.any? { |ext| path.end_with?(ext) }
         end
 

spec/lib/gitlab/static_site_editor/config/generated_config_spec.rb

@@ -58,25 +58,9 @@ RSpec.describe Gitlab::StaticSiteEditor::Config::GeneratedConfig do
         )
       end
 
-      context 'when feature flag is enabled' do
-        let(:path) { 'FEATURE_ON.md.erb' }
+      let(:path) { 'README.md.erb' }
 
-        before do
-          stub_feature_flags(sse_erb_support: project)
-        end
-
-        it { is_expected.to include(is_supported_content: true) }
-      end
-
-      context 'when feature flag is disabled' do
-        let(:path) { 'FEATURE_OFF.md.erb' }
-
-        before do
-          stub_feature_flags(sse_erb_support: false)
-        end
-
-        it { is_expected.to include(is_supported_content: false) }
-      end
+      it { is_expected.to include(is_supported_content: true) }
     end
 
     context 'when file path is nested' do