BUG/CLN: fix iloc when positional indexer matched Int64Index key

closes #6612

As discussed in #6612, `reindex(..., takeable=True)` as released in 0.13.0 didn't do anything that's not achieved by `.take`, `.iloc` and `.copy` attrs/methods, but also contained a bug which propagated to `iloc` when the latter was changed to use *reindex/takeable* internally.

The proposed solution is to remove *reindex/takeable* functionality altogether. According to @jreback, it was never supposed to be a part of public API, so probably no deprecation procedure is necessary.