Clean up unnecessary files under js/src

[eason9487]

Changes proposed in this Pull Request:

Noticed a few unused files when working on #2711.

This PR removes them along with the test case of trialing Jest tests with @testing-library/react.

💡 I will be skipping code review since this PR only includes developer-facing only change.

Detailed test instructions:

Confirm the npm run dev can build frontend code without errors

Changelog entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.4%. Comparing base (7cd33a8) to head (2d36ead).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2712     +/-   ##
=========================================
- Coverage     63.4%   63.4%   -0.0%     
=========================================
  Files          340     339      -1     
  Lines         5210    5207      -3     
  Branches      1275    1275             
=========================================
- Hits          3305    3302      -3     
  Misses        1730    1730             
  Partials       175     175             

Flag	Coverage Δ
js-unit-tests	63.4% <ø> (-<0.1%)	⬇️

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