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

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

lib/gitlab/wiki_pages/front_matter_parser.rb

@@ -3,8 +3,6 @@
 module Gitlab
   module WikiPages
     class FrontMatterParser
-      FEATURE_FLAG = :wiki_front_matter
-
       # We limit the maximum length of text we are prepared to parse as YAML, to
       # avoid exploitations and attempts to consume memory and CPU. We allow for:
       #  - a title line
@@ -30,18 +28,12 @@ def initialize(content:, front_matter: {}, reason: nil, error: nil)
       end
 
       # @param [String] wiki_content
-      # @param [FeatureGate] feature_gate The scope for feature availability
-      def initialize(wiki_content, feature_gate)
+      def initialize(wiki_content)
         @wiki_content = wiki_content
-        @feature_gate = feature_gate
-      end
-
-      def self.enabled?(gate = nil)
-        Feature.enabled?(FEATURE_FLAG, gate)
       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)
@@ -94,16 +86,12 @@ def not_mapping?
 
       private
 
-      attr_reader :wiki_content, :feature_gate
+      attr_reader :wiki_content
 
       def empty_result(reason = nil, error = nil)
         Result.new(content: wiki_content, reason: reason, error: error)
       end
 
-      def enabled?
-        self.class.enabled?(feature_gate)
-      end
-
       def block
         @block ||= parse_front_matter_block
       end