Dimension Leak Fix

v1.0.2:
- Fixes a crash when travelling through a portal alongside mods that keep a reference to an entity, such as Perception. Reported by a user, and the fault was mine.
- Until 1.0.1 the cleanup set Entity.level to null on every entity of the world you were leaving, to break the entity-to-world chain that keeps the old world in memory. Vanilla guarantees that field is never null while the entity lives, so nothing guards against it: any mod holding an entity across the transition read null and crashed. That step is gone.
- Nothing else changes. The deferred pass still empties the old world's own containers, which needs no such mutation, and the leak the removed step targeted belongs to whichever mod retains the entity in the first place.