Remove improved_merge_diff_highlighting feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

• Introduced in !52499 (merged)
• FF issue: #299884

Remove the feature flag and the deprecated diff detection logic. The feature flag was enabled by default in 13.9.

Does this MR meet the acceptance criteria?

Conformity

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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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done