Error while deleting/dropping a series/dataframe entry with timezone aware timestamps.
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
start = pd.Timestamp('20170101', tz='Europe/Berlin')
end = pd.Timestamp('20180101', tz='Europe/Berlin')
index = pd.date_range(start, end, freq='15min')
data = [1 for x in range(len(index))]
series = pd.Series(index=index, data=data)
dataframe = pd.DataFrame(series)
timestamp = pd.Timestamp('201701011515', tz='Europe/Berlin')

# Working
series[timestamp]
dataframe.loc[timestamp]

# Errors
del series[timestamp]
dataframe.drop(timestamp)
series.drop(timestamp)

```
#### Problem description

It not possible to delete/drop a series/dataframe entry with timezone aware timestamps, while accessing them is possible.

#### Expected Output

Deletion of the respective entry.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.1
pytest: 3.6.2
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
