Remove compute function.

[ueshin]

I don't think anyone is using compute() function.

[codecov-commenter]

Codecov Report

Merging #1531 into master will increase coverage by 0.11%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #1531      +/-   ##
==========================================
+ Coverage   93.98%   94.09%   +0.11%     
==========================================
  Files          36       36              
  Lines        8489     8396      -93     
==========================================
- Hits         7978     7900      -78     
+ Misses        511      496      -15     

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	92.15% <ø> (-0.16%)	⬇️
databricks/koalas/frame.py	95.43% <ø> (ø)
databricks/koalas/generic.py	95.28% <ø> (-1.48%)	⬇️
databricks/koalas/typedef/typehints.py	85.26% <86.66%> (+4.18%)	⬆️
databricks/koalas/typedef/__init__.py	100.00% <100.00%> (+15.78%)	⬆️

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