Remove subsection for Databricks Runtime in Getting Started

[HyukjinKwon]

[codecov-io]

Codecov Report

Merging #964 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #964      +/-   ##
==========================================
+ Coverage   94.52%   94.57%   +0.04%     
==========================================
  Files          34       34              
  Lines        6466     6483      +17     
==========================================
+ Hits         6112     6131      +19     
+ Misses        354      352       -2

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <0%> (ø)	⬆️
databricks/koalas/missing/indexes.py	100% <0%> (ø)	⬆️
databricks/koalas/plot.py	94.28% <0%> (ø)	⬆️
databricks/koalas/missing/series.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	96.03% <0%> (ø)	⬆️
databricks/koalas/indexes.py	96.47% <0%> (+0.02%)	⬆️
databricks/koalas/base.py	94.9% <0%> (+0.06%)	⬆️
databricks/koalas/series.py	96.32% <0%> (+0.22%)	⬆️
databricks/koalas/namespace.py	87.14% <0%> (+0.31%)	⬆️

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[HyukjinKwon]

@rxin, please revert this right away (by clicking the button below) if you don't like .. I don't mind at all.