Simplify plot backend support

[HyukjinKwon]

This PR proposes to simplify plot implementation. Current Koalas implementation attempts to map the argument between other plotting backends (e.g., matplotlib vs plotly). Keeping this map is a huge maintenance cost and it's unrealistic to track their change and keep updating this map.

Pandas itself does not keep this map either:

>>> import pandas as pd
>>> pd.DataFrame([1,2,3]).plot.line(logx=1)
<AxesSubplot:>
>>> pd.options.plotting.backend = "plotly"
>>> pd.DataFrame([1,2,3]).plot.line(logx=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../opt/miniconda3/envs/python3.8/lib/python3.8/site-packages/pandas/plotting/_core.py", line 1017, in line
    return self(kind="line", x=x, y=y, **kwargs)
  File "/.../opt/miniconda3/envs/python3.8/lib/python3.8/site-packages/pandas/plotting/_core.py", line 879, in __call__
    return plot_backend.plot(self._parent, x=x, y=y, kind=kind, **kwargs)
  File "/.../miniconda3/envs/python3.8/lib/python3.8/site-packages/plotly/__init__.py", line 102, in plot
    return line(data_frame, **kwargs)
TypeError: line() got an unexpected keyword argument 'logx'

[HyukjinKwon]

I think this mapping was copied from pandas, which is actually not needed in our case since we're dispatching it in each method e.g.) KoalasPlotAccessor.area.

[codecov-io]

Codecov Report

Merging #1970 (d844086) into master (d1babcc) will decrease coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1970      +/-   ##
==========================================
- Coverage   94.60%   94.58%   -0.02%     
==========================================
  Files          49       49              
  Lines       10890    10866      -24     
==========================================
- Hits        10302    10278      -24     
  Misses        588      588              

Impacted Files	Coverage Δ
databricks/koalas/config.py	99.00% <ø> (ø)
databricks/koalas/plot/core.py	91.54% <91.66%> (-1.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d1babcc...d844086. Read the comment docs.

[ueshin]

LGTM.

[ueshin]

Thanks! merging.

[xinrong-meng]

Awesome!