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

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app/models/wiki_page.rb

@@ -326,7 +326,7 @@ def update_front_matter(attrs)
 
   def parsed_content
     strong_memoize(:parsed_content) do
-      Gitlab::WikiPages::FrontMatterParser.new(raw_content, container).parse
+      Gitlab::WikiPages::FrontMatterParser.new(raw_content).parse
     end
   end