Resolve "[Feature flag] Rollout of `increase_diff_file_performance`"

What does this MR do and why?

This feature flag has been turned on for 2 months now - Roll out issue #432600 (closed)

Turning this FF on passes a smaller blob size limit for merge request file collections.

The issue before this feature flag, was that Gitaly was timing out when we were requesting a very large amount of data, but we didn't use this data so didn't need to request it.