Remove :display_merge_conflicts_in_diff Feature Flag

What does this MR do and why?

Describe in detail what your merge request does and why.

The last feature being developed behind this flag ( !67944 (merged) ) is now complete, so this can be removed and enabled by default

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #276918 (closed)