FTI - Clear SceneTreeFTI completely on enabling / disabling.

[lawnjelly]

Removes any dangling objects when toggling set_enabled() for SceneTreeFTI.
These could cause crashes / errors if deleted between toggling the setting.

MRP

Run godotengine/godot-demo-projects#1223 and keep toggling physics interpolation by pressing T while firing.. the DEV_ASSERT() will hit.

Notes

• Sorry my bad, I forgot to clear these completely when adding more lists.
• Should be very safe, and worth doing for 4.5.
• Also applicable in 3.x.

[rburing]

Makes total sense, and indeed will be good for 4.5.

[Repiteo]

Thanks!