Inside stmts module,
somewhere we read, `self.groupby` attribute being added if not exists and used as a list:
```
if not self.groupby:
    self.groupby = []
```

Let's just define:
- self.groupby self.orderby as instance attribute
- self.groupby self.orderby as List of Any
- remove the if not self.grouby check

Do the same with the `self.orderby` attribute.

Inside Select class:
- update `remove` method signature. And fix the error:
  rql/stmts.py:940: error:
  Argument 1 to "remove_sort_term" of "Select" has incompatible type "BaseNode";
  expected "SortTerm"  [arg-type]

- update undefine_variable method:
  remove useless check of var.variable attribute

Inside undo.NodeOperation class:
- simplify type hint for `node` parameter inside `NodeOperation.__init__` method