[OOPPD] Simplify DuplicateChecker

Apply all the unaddressed suggestions from [1] to clean up the code.

- Change class with no members into a function.
- Use a range-based for-loop.
- Use std::set<T> instead of std::map<T, bool> to track seen items.
- Don't bother with find() and set, just insert().

[1] https://crrev.com/c/2387545/19/printing/backend/mojom/print_backend_mojom_traits.cc#57

Change-Id: I74bd69154983dc854ab2248f11156a52a829da59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6488840
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1452025}