BUG: Get_period_alias(offset_str) function behaves inconsistent
`Get_period_alias(offset_str)` function behaves inconsistent
---

```python

import pandas as pd

print(pd.tseries.frequencies.get_period_alias('AS-JAN')) # works fine
print(pd.tseries.frequencies.get_period_alias('AS-FEB')) # works fine
# ...
print(pd.tseries.frequencies.get_period_alias('W-MON')) # offset not removed
print(pd.tseries.frequencies.get_period_alias('W-TUE')) # offset not removed
# ...
print(pd.tseries.frequencies.get_period_alias('A-JAN')) # offset not removed
print(pd.tseries.frequencies.get_period_alias('A-FEB')) # offset not removed
# ...

```

#### Problem description

The `Get_period_alias(offset_str)` behaves inconsistent for different frequencies. See code sample for examples. The offset information (JAN, FEB, MON, TUE, ... ) should be removed. The issue should be easy to resolve by modifying the following lines:

https://github.com/pandas-dev/pandas/blob/1f6710035a5295b2d3171ab811ae7ffca2344389/pandas/tseries/frequencies.py#L81-L87

#### Output:
```
A
A
W-MON
W-TUE
A-JAN
A-FEB
```

#### Expected Output
```
A
A
W
W
A
A
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.7.0.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.17763
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.3
numpy            : 1.16.3
pytz             : 2018.5
dateutil         : 2.7.3
pip              : 19.1.1
setuptools       : 50.3.0
Cython           : 0.28.5
pytest           : 4.0.1
hypothesis       : None
sphinx           : 2.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.1.0
lxml.etree       : 4.2.5
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 6.5.0
pandas_datareader: None
bs4              : 4.6.3
bottleneck       : 1.2.1
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 2.2.3
numexpr          : 2.6.8
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : 1.3.13
tables           : 3.4.4
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.39.0

</details>
