Removed the Project.undos variable

[Variable-ind]

An old commit for 0.4: 8fadacd introduced theundos variable at 3 important places in Canvas.gd

These were later removed/replaced by better alternatives (see below)
This PR removes all the other remaining traces of undos variable.

(The 3 important places and the commits that remove them)

---

# If we're already pressing a mouse button and we haven't handled undo yet,...
    #. .. it means that the cursor was outside the canvas. Then, ...
    # simulate "just pressed" logic the moment the cursor gets inside the canvas

    # Or, if we're making a line. This is used for handling undo/redo for lines...
    # ...that go outside the canvas
    if Input.is_action_pressed("left_mouse") || Input.is_action_pressed("right_mouse"):
        if (mouse_in_canvas && Global.undos < Global.undo_redo.get_version()) || is_making_line:
            mouse_pressed = true

The need for this section was removed by bda9e62#diff-fda2b27bbbd524f0edab9f12f5a93963e7eb070bebc1672ad9d3dbc144d2560e

---

func handle_redo(
    _action: String, project: Project = Global.current_project, layer_index := -2, frame_index := -2
) -> void:
    can_undo = true
    if project.undos < project.undo_redo.get_version():
        return

The need for this segment was removed by f7ef4a4

---

        if can_handle || current_project.undos == current_project.undo_redo.get_version():
            if previous_action != -1 && previous_action != Global.Tools.RECTSELECT && current_action != Global.Tools.COLORPICKER && current_action != Global.Tools.ZOOM:
                handle_redo("Draw")

    handle_tools(current_mouse_button, current_action, mouse_pos, can_handle)

The need for this segment was removed by #281 (sub-commit 789b2b0bf181f324655cbd9b6313477109af596a)

[OverloadedOrama]

Thanks!