[Python][C++] index_in_meta_binary/is_in_meta_binary should not show up in functions list

The index_in_meta_binary function is an implementation detail and should not show up in the list of functions.

The same also applies to is_in_meta_binary

**Reporter**: [Weston Pace](https://issues.apache.org/jira/browse/ARROW-13949) / @westonpace
#### Related issues:
- [[Python] Hide hash_* functions from pyarrow.compute](https://github.com/apache/arrow/issues/29552) (is related to)
- [[C++] Duplicate functions array_filter/array_take and filter/take](https://github.com/apache/arrow/issues/18821) (is related to)

_{**Note**: *This issue was originally created as [ARROW-13949](https://issues.apache.org/jira/browse/ARROW-13949). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

No files or tests are named in the issue. Locate where index_in_meta_binary and is_in_meta_binary are registered for the functions list, then verify that both remain usable internally but no longer appear in the exposed list; add or update coverage for that behavior.

Written by the indexing model from the issue text.