Remove display_merge_conflicts_in_diff feature flag
What does this MR do and why?
The feature flag has been enabled by default for all instances in 15.7 and no reports/issues were reported.
This removes the feature flag.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #276918 (closed)