Remove wiki_front_matter FF

What does this MR do and why?

A small MVC for: #353926 (closed) to add the feature by default.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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spec/models/wiki_page_spec.rb

@@ -40,12 +40,6 @@ def wiki
       it { expect(wiki_page).to have_attributes(front_matter: {}, content: content) }
     end
 
-    shared_examples 'a page with front-matter' do
-      let(:front_matter) { { title: 'Foo', slugs: %w[slug_a slug_b] } }
-
-      it { expect(wiki_page.front_matter).to eq(front_matter) }
-    end
-
     context 'the wiki page has front matter' do
       let(:content) do
         <<~MD
@@ -60,8 +54,9 @@ def wiki
         MD
       end
 
-      it_behaves_like 'a page with front-matter'
+      let(:front_matter) { { title: 'Foo', slugs: %w[slug_a slug_b] } }
 
+      it { expect(wiki_page.front_matter).to eq(front_matter) }
       it 'strips the front matter from the content' do
         expect(wiki_page.content.strip).to eq('My actual content')
       end