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`

pls asv indexing (shouldn't be an issue, but just check). Turns out get_value_box is necessary because it raises `IndexError` in cases where the other version ignores it.  So now the change is just to move `get_value_box` from libts to libindex. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=h1) Report
> Merging [#18371](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b00e62c273cd60739b538314156b65f0f79350f9?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18371/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18371      +/-   ##
==========================================
- Coverage   91.38%   91.36%   -0.03%     
==========================================
  Files         164      164              
  Lines       49790    49799       +9     
==========================================
- Hits        45501    45499       -2     
- Misses       4289     4300      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.16% <100%> (-0.01%)` | :arrow_down: |
| #single | `39.55% <100%> (-0.04%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.42% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.52% <0%> (-0.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.54% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.92% <0%> (+0.01%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=footer). Last update [b00e62c...38e29d1](https://codecov.io/gh/pandas-dev/pandas/pull/18371?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! @jbrockmendel a general comment: can you use the 'CLN: ' tag in PR titles / commits for PRs that are purely cleaning things up? (so no bug, enhancement or performance related aspect)