users: don't log mod actions for account upgrades.

Account upgrades are now logged on the /user_upgrades page, so they
no longer need to be recorded as mod actions. The mod actions log should
be reserved for privileged actions performed by Builders and above. They
also tended to spam the mod actions log.

app/models/mod_action.rb

@@ -20,7 +20,7 @@ class ModAction < ApplicationRecord
     user_level_change: 7,
     user_approval_privilege: 8,
     user_upload_privilege: 9,
-    user_account_upgrade: 19,
+    user_account_upgrade: 19, # XXX unused
     user_feedback_update: 21,
     user_feedback_delete: 22,
     post_delete: 42,

app/models/user_upgrade.rb

@@ -108,7 +108,6 @@ class UserUpgrade < ApplicationRecord
 
         if payment_status == "paid"
           upgrade_recipient!
-          create_mod_action!
           dmail_recipient!
           dmail_purchaser!
           update!(status: :complete)
@@ -122,10 +121,6 @@ class UserUpgrade < ApplicationRecord
       recipient.update!(level: level)
     end
 
-    def create_mod_action!
-      ModAction.log(%{"#{recipient.name}":#{Routes.user_path(recipient)} level changed #{User.level_string(recipient.level_before_last_save)} -> #{recipient.level_string}}, :user_account_upgrade, purchaser)
-    end
-
     def dmail_recipient!
       if is_gift?
         body = "Congratulations, your account has been upgraded to #{level_string} by <@#{purchaser.name}>. Enjoy!"