Remove api_v3_commits_skip_diff_files feature flag

What does this MR do and why?

This MR removes the api_v3_commits_skip_diff_files feature flag that has been default enabled since %14.5 in !74355 (merged).

• Issue: #344617 (closed)

MR acceptance checklist

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

Related to #344617 (closed)