API: implement droplevels() for flat index

[toobaz]

Code Sample, a copy-pastable example if possible

In [2]: pd.Index([1, 2, 3]).droplevel([])
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-9172aa27c42a> in <module>()
----> 1 pd.Index([1, 2, 3]).droplevel([])

AttributeError: 'Int64Index' object has no attribute 'droplevel'

Problem description

Clearly the call above doesn't make too much sense, but there is no reason not to support it, and can help write cleaner code.

Expected Output

In [2]: pd.Index([1, 2, 3]).droplevel([])
Out[2]: Int64Index([1, 2, 3], dtype='int64')

Output of pd.show_versions()

INSTALLED VERSIONS

commit: e033c06
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+10.ge033c0616.dirty
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.0.1
Cython: 0.25.2
numpy: 1.14.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
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

[toobaz]

By the way: I'm sure there was an issue with a list of MultiIndex methods that should have been ported to Index too... but I wasn't able to find it.

[jschendel]

issue with a list of MultiIndex methods that should have been ported to Index

xref #3268, which includes droplevel