Revert "Merge branch '353926' into 'master'"

What does this MR do and why?

Revert !81833 (merged) due to the feature causing issues for Wiki Frontmatter. See #359081 (closed). For now, we will re-instate the feature flag until we can invest time in fixing the root bug. That way those impacted by the issue have a workaround until then.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

app/models/wiki_page.rb

@@ -316,6 +316,7 @@ 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])
@@ -326,7 +327,7 @@ def update_front_matter(attrs)
 
   def parsed_content
     strong_memoize(:parsed_content) do
-      Gitlab::WikiPages::FrontMatterParser.new(raw_content).parse
+      Gitlab::WikiPages::FrontMatterParser.new(raw_content, container).parse
     end
   end