fix(plugin-chart-pivot-table): make date formatting clearable

[kgabryje]

Before there was no way to disable date formatting to see the original value of temporal column in pivot table. This PR enables clearing the select button value.

Screen.Recording.2021-08-02.at.18.00.12.mov

Fixes apache/superset#15947

CC @junlincc @jinghua-qa

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[codecov]

Codecov Report

Merging #1261 (4470712) into master (8012a6c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1261   +/-   ##
=======================================
  Coverage   30.04%   30.04%           
=======================================
  Files         476      476           
  Lines        9666     9669    +3     
  Branches     1596     1598    +2     
=======================================
+ Hits         2904     2905    +1     
  Misses       6538     6538           
- Partials      224      226    +2     

Impacted Files	Coverage Δ
...lugin-chart-pivot-table/src/plugin/controlPanel.ts	12.50% <ø> (ø)
...ins/plugin-chart-echarts/src/Tree/controlPanel.tsx	50.00% <0.00%> (-10.00%)	⬇️
...et-ui-chart-controls/src/shared-controls/index.tsx	36.45% <0.00%> (-0.39%)	⬇️
...chart-controls/src/shared-controls/dndControls.tsx	38.88% <0.00%> (+1.74%)	⬆️

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