[Fix] Restore ONCE_RECEIVED symbol

Issue: #602
Reported by: @AlexeySa

src/plugins/lua/once_received.lua

@@ -147,7 +147,6 @@ if opts then
     local id = rspamd_config:register_symbol({
       name = symbol,
       callback = check_quantity_received,
-      type = 'callback',
     })
 
     for n,v in pairs(opts) do