remove get_value_box since it is redundant with libindex.get_value_at
`tslib.get_value_box` is effectively identical to `index.get_value_at` (figuring this out requires looking at `util.get_value_at`).  Since the only use of `get_value_box` is in `core.index.base`, just get rid of it and use `libindex.get_value_at` there instead.

Fixup type declarations in `util.get_value_at`.

- [x] closes #18299
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
