Remove Index.__inv__
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd

In [2]: idx = pd.Index([2, 4, 6])

In [3]: idx.__inv__()
Out[3]: Int64Index([-2, -4, -6], dtype='int64')
```
#### Problem description

``idx.__inv__`` does what ``idx.__neg__`` does, and not do what its name suggests (that is, ``operator.inv``). By the way, ``~idx`` raises ``TypeError`` anyway, and ``idx.__invert__`` does not exist, differently from what [the Python docs suggest](https://docs.python.org/2/library/operator.html).

Notice that instead
```
In [4]: s = pd.Series([2, 4, 6])

In [5]: ~s
Out[5]: 
0   -3
1   -5
2   -7
dtype: int64

In [6]: s.__invert__()
Out[6]: 
0   -3
1   -5
2   -7
dtype: int64
```
which is correct (and ``Series.__inv__`` does not exist).

#### Expected Output

```
Out[3]: Int64Index([-3, -5, -7], dtype='int64')
```

Given how wrong, undocumented and not exposed this is, I suggest to immediately remove ``Index.__inv__``. Then we can later decide to introduce ``Index.__invert__`` which does the right thing, and also enable the ``~`` operator.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+460.geb0ac5437
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.28.4
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1634.dev0+ge8120cf6d
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1
gcsfs: None


</details>
