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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lib/gitlab/wiki_pages/front_matter_parser.rb

@@ -33,7 +33,7 @@ def initialize(wiki_content)
       end
 
       def parse
-        return empty_result unless enabled? && wiki_content.present?
+        return empty_result unless wiki_content.present?
         return empty_result(block.error) unless block.valid?
 
         Result.new(front_matter: block.data, content: strip_front_matter_block)