Remove default_merge_ref_for_diffs feature flag

What does this MR do and why?

The feature has been enabled by default for a while now and no blocking issues were found.

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 #276917 (closed)