Series.drop() on series with timezone aware index can yield AmbiguousTimeError
#### Code Sample, a copy-pastable example if possible

```python
import datetime
import pandas as pd

series = pd.Series(
    list(range(8760)),
    index=pd.date_range('2018/01/01', periods=8760, freq='H', tz='Europe/Berlin')
)

# trying to drop first day
series = series.drop(
    series.index[series.index < datetime.datetime(year=2018, month=1, day=2)]
)

# yields AmbiguousTimeError 

```

```
File "pandas/_libs/tslib.pyx", line 3593, in pandas._libs.tslib.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from Timestamp('2018-10-28 02:00:00'), try using the 'ambiguous' argument
```

#### Problem description
I can't use `drop` on series that have a timezone aware datetime index. For me, it is not understandable why the error displayed above is raised. 

The workaround I found is:
* find the positions to drop
* convert the series to UTC
* drop
* convert back to original timezone

```python
import datetime
import pandas as pd

series = pd.Series(
    list(range(8760)),
    index=pd.date_range('2018/01/01', periods=8760, freq='H', tz='Europe/Berlin')
)

positions_to_drop = series.index < datetime.datetime(year=2018, month=1, day=2)

tz = series.index.tz
series.index = series.index.tz_convert('UTC')
series = series.drop(
    series.index[positions_to_drop]
)
series.index = series.index.tz_convert(tz)
```

#### Expected Output
No error. And the result the previous snippet yields.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None
LOCALE: de_DE.UTF-8
pandas: 0.22.0
pytest: 3.4.2
pip: 9.0.1
setuptools: 38.5.2
Cython: None
numpy: 1.14.2
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: 1.7.1
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.4
feather: None
matplotlib: None
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
