Unbound: Remove backup file brought in from upstream

Upstream tarball included config.h~.

This was previously manually removed from vendor where in fact it
should not have been as it would have caused a merge conflict.

Reported by: "Herbert J. Skuhra" <herbert@gojira.at>

(cherry picked from commit 5aab39b24ce7437265b94461ffdd9b12c0723658)