Remove "diff_line_syntax_highlighting" feature flag

What does this MR do and why?

Contributes to #324159 (closed)

Feature flag diff_line_syntax_highlighting is disabled for too long. It's better to remove it completely from the codebase.

References

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• [Feature flag] Rollout of `diff_line_syntax_hig... (#324159 - closed)

Related to #324159 (closed)