Refactor access patterns of `UnsafeWorldCell`

> Not super comfortable with this, since the rest of `UnsafeWorldCell`'s method don't use or respect this memory access pattern (`world_mut` is a special case). They just create and encapsulate `&World` references and provide restricted access. It's fine for now though since it's private and unsafe, but I think we should refactor the rest of `UnsafeWorldCell` to use this memory access pattern, and probably remove `world_metadata` since it's possible to access metadata safely through provided methods.

_Originally posted by @JoJoJet in https://github.com/bevyengine/bevy/pull/10756#discussion_r1480976815_