Avoid unneeded reset_index in DataFrameGroupBy.describe.

[ueshin]

DataFrameGroupBy.describe uses reset_index() internally, but we can avoid using it and so unnecessary default index attachment.

[codecov-io]

Codecov Report

Merging #1951 (1e6f3ea) into master (138c7b8) will decrease coverage by 3.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1951      +/-   ##
==========================================
- Coverage   94.64%   91.19%   -3.46%     
==========================================
  Files          49       49              
  Lines       10818    10713     -105     
==========================================
- Hits        10239     9770     -469     
- Misses        579      943     +364     

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.35% <100.00%> (-0.07%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	26.58% <0.00%> (-73.42%)	⬇️
databricks/koalas/usage_logging/__init__.py	25.66% <0.00%> (-66.65%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	37.50% <0.00%> (-58.80%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	⬇️
databricks/koalas/__init__.py	68.75% <0.00%> (-21.88%)	⬇️
databricks/conftest.py	89.83% <0.00%> (-10.17%)	⬇️
databricks/koalas/spark/functions.py	88.88% <0.00%> (-7.41%)	⬇️
databricks/koalas/accessors.py	86.43% <0.00%> (-7.04%)	⬇️
databricks/koalas/namespace.py	78.17% <0.00%> (-6.07%)	⬇️
... and 20 more

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[xinrong-meng]

Make senses! Thank you!

[ueshin]

Thanks! merging.

[itholic]

Nice fix