Remove flush_and_reserve_invalid_assuming_no_entities

[SpecificProtagonist]

Objective

flush_and_reserve_invalid_assuming_no_entities was made for the old rendering world (which was reset every frame) and is usused since the 0.15 retained rendering world, but wasn't removed yet. It is pub, but is undocumented apart from the safety comment.

Solution

Remove flush_and_reserve_invalid_assuming_no_entities and the safety invariants this method required for EntityMeta, EntityLocation, TableId and TableRow. This reduces the amount of unsafe code & safety invariants and makes #16047 easier.

Alternatives

• Document flush_and_reserve_invalid_assuming_no_entities and keep it unchanged
• Document flush_and_reserve_invalid_assuming_no_entities and change it to be based on EntityMeta::INVALID

Migration Guide

• exchange Entities::flush_and_reserve_invalid_assuming_no_entities for reserve and flush_as_invalid and notify us if that's insufficient

[alice-i-cecile]

I feel quite strongly that we should get rid of this. Keeping around unsafe backdoors with no users is just asking for surprising problems. For the migration guide, I mostly want them to tell us why they were using this!