DEV: Remove reference to non-existent `letter_proxy` route

This was introduced in f04471e422, but never actually used

app/controllers/user_avatars_controller.rb

@@ -37,7 +37,7 @@ class UserAvatarsController < ApplicationController
   def show_proxy_letter
     is_asset_path
 
-    if SiteSetting.external_system_avatars_url !~ /^\/letter(_avatar)?_proxy/
+    if SiteSetting.external_system_avatars_url !~ /^\/letter_avatar_proxy/
       raise Discourse::NotFound
     end