AmbiguousTimeError using drop on DatetimeIndex with DST change
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
start = pd.Timestamp('201710290100', tz='Europe/Brussels')
end = pd.Timestamp('201710290300', tz='Europe/Brussels')
index = pd.date_range(start=start, end=end, freq='15min')

index.drop(index[0])

AmbiguousTimeError: Cannot infer dst time from Timestamp('2017-10-29 02:00:00'), try using the 'ambiguous' argument
```
#### Problem description

When dropping from a localized DateTimeIndex, behind the scenes the index is converted to UTC, the drop is performed, and then it is converted back to the original timezone. (This is what I make of the traceback).
This second conversion is ambiguous because the original index contains a DST change.

#### Expected Output

#### Output of ``index.drop(index[0])``

<details>

> DatetimeIndex(['2017-10-29 01:15:00+02:00', '2017-10-29 01:30:00+02:00',
               '2017-10-29 01:45:00+02:00', '2017-10-29 02:00:00+02:00',
               '2017-10-29 02:15:00+02:00', '2017-10-29 02:30:00+02:00',
               '2017-10-29 02:45:00+02:00', '2017-10-29 02:00:00+01:00',
               '2017-10-29 02:15:00+01:00', '2017-10-29 02:30:00+01:00',
               '2017-10-29 02:45:00+01:00', '2017-10-29 03:00:00+01:00'],
              dtype='datetime64[ns, Europe/Brussels]', freq='15T')

</details>
