Fix Joint2D/Joint3D node path reset on scene switch

[pouleyKetchoupp]

When one of the bodies exited the tree, the corresponding node path was reset instead of just resetting the joint from the physics server. That was causing the node path to be reset on scene switch when one of the bodies is under the joint in the scene tree.

Regression from #44703
Fixes #46571
Tested with case from #44510, still fixed.

[akien-mga]

The change seems to invalidate most of the rest of what was added in #44703 (connecting signals, then adding code in several places to disconnect the signals). If the proposed change is sufficient, then the previously added code could likely be removed too and replaced by connecting the body's tree_exited signal directly to update_joints(true), no?

[pouleyKetchoupp]

@akien-mga Let me know if I'm missing something, but it seems still safer to disconnect signals from both bodies when one of them exits the tree to avoid any side effect.

So I would rather keep _body_exit_tree which disconnects signals and disables the joint, but I've just updated the PR to remove the body id that was passed as an argument since it's not used anymore.

[akien-mga]

Thanks!