Clean up the backend

[yixuy]

Description

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How Has This Been Tested?

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Snapshots:

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Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have already squashed the commits and make the commit message conform to the project standard.
• [ ] I have already marked the commit with "BREAKING CHANGE" or "Fixes #" if needed.
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[codecov[bot]]

Codecov Report

Merging #832 (c187c34) into develop (482ca40) will decrease coverage by 0.61%. The diff coverage is 7.71%.

@@             Coverage Diff             @@
##           develop     #832      +/-   ##
===========================================
- Coverage    55.32%   54.70%   -0.62%     
===========================================
  Files          293      293              
  Lines        18993    19224     +231     
===========================================
+ Hits         10507    10516       +9     
- Misses        8486     8708     +222     

Impacted Files	Coverage Δ
dataprep/clean/clean_url.py	97.84% <ø> (ø)
dataprep/clean/gui/clean_gui.py	11.50% <0.00%> (-9.21%)	:arrow_down:
dataprep/eda/create_diff_report/__init__.py	50.00% <0.00%> (ø)
dataprep/eda/create_diff_report/diff_formatter.py	16.58% <0.00%> (ø)
dataprep/eda/correlation/render.py	98.69% <100.00%> (+0.02%)	:arrow_up:
dataprep/eda/distribution/render.py	90.73% <100.00%> (ø)
dataprep/tests/eda/test_create_report.py	87.87% <100.00%> (+0.78%)	:arrow_up:
dataprep/tests/eda/test_plot.py	100.00% <100.00%> (ø)
... and 2 more

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