DEV: Remove translate API from plugin.

It appears to be completely unused at this point.

lib/plugin/instance.rb

@@ -30,15 +30,6 @@ class Plugin::CustomEmoji
   def self.unregister(name, group = Emoji::DEFAULT_GROUP)
     emojis[group].delete(name)
   end
-
-  def self.translations
-    @@translations ||= {}
-  end
-
-  def self.translate(from, to)
-    @@cache_key = Digest::SHA1.hexdigest(cache_key + from)[0..10]
-    translations[from] = to
-  end
 end
 
 class Plugin::Instance
@@ -491,10 +482,6 @@ class Plugin::Instance
     Emoji.clear_cache
   end
 
-  def translate_emoji(from, to)
-    Plugin::CustomEmoji.translate(from, to)
-  end
-
   def automatic_assets
     css = styles.join("\n")
     js = javascripts.join("\n")