Remove highlighting conflicts on diff

What does this MR do and why?

Since it's currently confusing to see conflicts inline on the diff, we are removing the functionality to show conflicts on diff.

We still keep the functionality to show the conflict type alert (per file) on the diff though.

This is behind display_merge_conflicts_in_diff feature flag.

Related to #365823 (closed).

How to set up and validate locally

1. Enable display_merge_conflicts_in_diff.
2. Create a MR with conflict to target branch.
3. View the MR.
4. The MR should show the diff with an alert on the conflicting file saying that it has conflict.

MR acceptance checklist

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