Remove frontend code quality comparison

What does this MR do?

This MR removes the frontend code that used to do the comparison between the code quality JSON artifacts for an MR to display in the code quality MR widget (downloading the base and head report artifacts for the MR branch, parsing the reports into code quality violations, comparing the violations in a worker thread, and displaying the changed violations in the widget).

Now that we're certain that the new backend code quality report comparison is working properly and we can remove :codequality_backend_comparison feature flag, all of the report parsing and comparing work will be done by the backend, so this frontend code is no longer needed.

Screenshots

No visual changes

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because this MR only cleans up code that is already unreachable.
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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