unbound: Remove backup file

Upstream unbound includes a backup configure file which is distributed
in the upstream tarball. It must be created by their release process and
not deleted prior to packaging the tarball. I've received two emails so
far asking about it. Let's remove it so nobody else asks about it.

(cherry picked from commit 51c8a9c1be57b6750e7c64e1379e8c33bd0f02c1)