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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app/models/wiki_page.rb

@@ -316,7 +316,6 @@ def serialize_front_matter(hash)
   end
 
   def update_front_matter(attrs)
-    return unless Gitlab::WikiPages::FrontMatterParser.enabled?(container)
     return unless attrs.has_key?(:front_matter)
 
     fm_yaml = serialize_front_matter(attrs[:front_matter])