Fix #29568: Blender restores deleted scene

Fix #29568: Blender restores deleted scene

Special notifiers used for scene deletion which lead to undo pushes
after SCENE_OT_delete operator happening with scene still present in
the mainfile. That was a reason why operator redo used to restore
scene.

It's not so obvious why special notifier type is needed for scene
set and deletion -- it confuses undo system without having some
obvious advantages. Using "direct" scene deletion and setting
seems to be working fine so let's see if there'll be some issues
with this.