PERF: Deprecate casting of index of dates to DatetimeIndex
```python
In [5]: index = pd.Index([pd.Timestamp('2001'), pd.Timestamp('2002')], dtype=object)

In [6]: pd.Series(1, index=index).index
Out[6]: DatetimeIndex(['2001-01-01', '2002-01-01'], dtype='datetime64[ns]', freq=None)
```

This was the root cause of https://github.com/pandas-dev/pandas/pull/23591. Why are we doing that?

Note that this doesn't affect the case of `index=[pd.Timestamp(...), pd.Timestamp(...)]`, as that would have previously been converted to a DatetimeIndex. It seems be only when you have an Index of datetimes.

I'm going through our test cases that hit this now.