remove index column in SeriesGroupBy.nlargest
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
pdf = pd.DataFrame({
    'a': [1, 1, 1, 2, 2, 2, 3, 3, 3],
    'b': [1, 2, 2, 2, 3, 3, 3, 4, 4]}, columns=['a', 'b'])

pdf.groupby(['a'])['b'].nlargest(1)
a   
1  1    2
2  4    3
3  7    4
Name: b, dtype: int64
```
#### Problem description

It raise the index to column(1, 4, 7) above. But we dont need it most time.

#### Expected Output
```python
pdf.groupby(['a'])['b'].nlargest(1)
a   
1     2
2    3
3    4
```

#### Output of ``pd.show_versions()``
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.5.2.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: zh_CN.utf8
LANG: None
LOCALE: zh_CN.UTF-8

pandas: 0.24.2
pytest: 5.0.1
pip: 18.1
setuptools: 41.0.1
Cython: 0.29.10
numpy: 1.17.0
scipy: 1.1.0
pyarrow: 0.14.1
xarray: None
IPython: 7.2.0
sphinx: 2.1.2
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: 1.2.1
tables: 3.5.1
numexpr: 2.6.9
feather: 0.4.0
matplotlib: 3.1.1
openpyxl: 2.6.1
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.8
lxml.etree: 4.2.5
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.3.5
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
