ENH: tz_localize(None) allows to reset tz
Closes #7812. Allow `tz_localize(None)` for tz-aware `Timestamp` and `DatetimeIndex` to reset tz.
CC @rockg, @nehalecky 

```
t = pd.Timestamp('2014-01-01 09:00', tz='Asia/Tokyo')

# tz_localize resets tz holding current tz representation
t.tz_localize(None)
#2014-01-01 09:00:00

# tz_convert reset tz using UTC representation (no changes by this PR)
t.tz_convert(None)
#2014-01-01 00:00:00

idx = pd.date_range(start='2011-01-01 09:00', freq='H', periods=10, tz='Asia/Tokyo')

idx.tz_localize(None)
# <class 'pandas.tseries.index.DatetimeIndex'
# [2011-01-01 09:00:00, ..., 2011-01-01 18:00:00]
# Length: 10, Freq: H, Timezone: None

#  (no changes by this PR)
idx.tz_convert(None)
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2011-01-01 00:00:00, ..., 2011-01-01 09:00:00]
# Length: 10, Freq: H, Timezone: None
```
