Use direct access to backend classes

issue8836
review258421002

product.py

@@ -50,8 +50,7 @@ class CategoryAccount(metaclass=PoolMeta):
 
     @classmethod
     def __register__(cls, module_name):
-        TableHandler = backend.get('TableHandler')
-        exist = TableHandler.table_exist(cls._table)
+        exist = backend.TableHandler.table_exist(cls._table)
         if exist:
             table = cls.__table_handler__(module_name)
             exist &= table.column_exist('account_cogs')