Fix NavigationObstacle3D debug clear regression

[smix8]

Clears NavigationObstacle3D on tree exit.

When users would switch scenes the obstacle debug would persist because the is_inside_tree() check in the _update_debug() would still return true, making the debug update instead of removing it.

[fire]

The code makes sense, but I don't know how to test it. @smix8 is the area maintainer, so it's probably okay.

[smix8]

Added a desc, you just need to add a NavigationObstacle3D with a radius > 0 and switch scenes to see that it did not clear. That was a reg from the obstacle editor plugin rework #93059.

[GremlinsInGame]

Will this solve the issue where NavigationObstacle3D visuals(orange spheres) are rendered in the wrong scenes in the editor or is that a different(new) issue that I'm experiencing. Im using 4.4beta1. I can provide more information if needed, but i'm thinking this PR may fix it.

[smix8]

@GremlinsInGame This fixes that the debug was not cleared when the node would leave the SceneTree, aka you switch editor scenes. Because the editor uses the same visual scenario for all scenes it would previously render the debug in all open editor scenes that you would switch to.

[Repiteo]

Thanks!