tz_localize creates freq inconsistency on time offset change
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

index = pd.date_range("2019-3-31", freq="30T", periods=10, tz="Europe/London")
print(index)  # this has a freq
print(index.freq)
print(index.tz_localize(None))  # this shouldn't have it anymore, but it does
print(index.tz_localize(None).freq)
```
#### Problem description

`tz_localize(None)` doesn't check if the frequency is still consistent with the new localized index. In the case of Daylight Saving Time switch, the frequency of the index should become None (since duplicated/missing timestamps are created).

#### Expected Output

Achievable with:

```python
new_index = index.tz_localize(None)
new_index.freq = new_index.inferred_freq  # drop frequency if not inferrable
print(new_index)  # this is correct
print(new_index.freq)
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : 0.29.14
pytest           : 5.3.2
hypothesis       : 4.54.2
sphinx           : 2.3.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.6
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.10.2
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : 0.4.0
scipy            : 1.3.2
sqlalchemy       : 1.3.11
tables           : 3.6.1
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.6

</details>
