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Both major members of Mandriva Linux family today either use DNF
as the sole package manager (OpenMandriva Lx) or as a fully supported
co-primary package manager as part of a transition (Mageia).

There are no actively supported releases of either distribution where
DNF isn't available and functional, so to simplify things and eliminate
some potentially poorly tested cruft, let's just get rid of it.

Change-Id: I7aa39f4e93219c4807f03af8de2adbe8935926fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553249
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830276}