local-unbound: remove stray config.h.in backup

despite being ignored in our main .gitignore, such files can still be
merged from vendor branches.

Reviewed by: cy
Fixes: 8f76bb7dad (unbound: Vendor import 1.18.0)
MFC after: 2 weeks
X-MFC with: 8f76bb7dad
Differential Revision: https://reviews.freebsd.org/D41920

(cherry picked from commit 948e11aaf420cd7d493cc2e118cacc06d18653fe)