Remove cached_markdown_blob feature flag

It's been enabled by default for a while, we can remove it

Changelog: other

app/models/blob_viewer/markup.rb

@@ -14,9 +14,7 @@ module BlobViewer
       {}.tap do |h|
         h[:rendered] = blob.rendered_markup if blob.respond_to?(:rendered_markup)
 
-        if Feature.enabled?(:cached_markdown_blob, blob.project, default_enabled: true)
-          h[:cache_key] = ['blob', blob.id, 'commit', blob.commit_id]
-        end
+        h[:cache_key] = ['blob', blob.id, 'commit', blob.commit_id]
       end
     end
   end

config/feature_flags/development/cached_markdown_blob.yml

----
-name: cached_markdown_blob
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44300
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/263406
-milestone: '13.5'
-type: development
-group: group::source code
-default_enabled: true

spec/models/blob_viewer/markup_spec.rb

@@ -24,15 +24,5 @@ RSpec.describe BlobViewer::Markup do
         expect(subject.banzai_render_context.keys).to include(:rendered)
       end
     end
-
-    context 'when cached_markdown_blob feature flag is disabled' do
-      before do
-        stub_feature_flags(cached_markdown_blob: false)
-      end
-
-      it 'does not set cache_key key' do
-        expect(subject.banzai_render_context.keys).not_to include(:cache_key)
-      end
-    end
   end
 end