Revert "Temporarily disable Markdown caching"

This reverts commit d337d5e7.

lib/banzai/renderer.rb

 module Banzai
   module Renderer
-    CACHE_ENABLED = false
-
     # Convert a Markdown String into an HTML-safe String of HTML
     #
     # Note that while the returned HTML will have been sanitized of dangerous
@@ -20,7 +18,7 @@ module Banzai
       cache_key = context.delete(:cache_key)
       cache_key = full_cache_key(cache_key, context[:pipeline])
 
-      if cache_key && CACHE_ENABLED
+      if cache_key
         Rails.cache.fetch(cache_key) do
           cacheless_render(text, context)
         end