BUG: DatetimeIndex.delete with tz raises ValueError

Found and fixed ``DatetimeIndex.delete`` (#7302) results in ``ValueError`` when it has ``tz`` and ``freq`` is less than daily..
```
idx = pd.date_range(start='2011-01-01 09:00', periods=40, freq='H', tz='Asia/Tokyo')
idx.delete(0)
# ValueError: Inferred frequency None from passed dates does notconform to passed frequency H
```

``insert`` (#7299) and ``take`` don't have problems, but changed test case to use hourly frequencies to detect this kind of problem.
Using ``tslib.tz_convert`` is better to handle array. But it looks having a bug in DST I don't fully understand. I'll try to dig in.