remove `World::get_mut_by_id` and `World::get_by_id`

`World::get_by_id` has no reason to exist as `EntityRef::get_by_id` accomplishes the same thing as it returns `Ptr<'w>` not `Ptr<'_>`.
`World::get_mut_by_id` should be moved to `EntityMut` as `EntityMut::get_mut_by_id_inner` (name subject to debate although has precedent `Query` has `_inner` methods for stuff that is more flexible with its lifetimes.) It would likely also be a good idea to introduce `EntityMut::get_by_id_inner` (name subject to debate).