Remove preload_gitaly_data feature flag

What does this MR do?

This MR removes the preload_mr_diff_gitaly_data feature flag and makes Gitaly data preloading the default behavior for merge request diffs. This feature flag has been turned on for over a week.

Why was this MR needed?

The preload_mr_diff_gitaly_data feature flag was introduced to gradually roll out performance improvements for merge request diff processing. After successful rollout and validation, the feature flag is no longer needed and should be removed to simplify the codebase.

What are the relevant issue numbers?

Rollout issue: #558037 (closed)