Makes 'diff/content' partial use instance method

What does this MR do?

Changes app/views/projects/diffs/_content.html.haml to use DiffFile#viewer instance method (which has the exact same conditional)

def viewer
  rich_viewer || simple_viewer
end

I discovered this while working on a proposal for https://gitlab.com/gitlab-org/gitlab-ce/issues/52499

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content