release: remove inet blackhole sysctls from GCE config

Other cloud images do not do this, and it can produce confusing results.

Reviewed by: Jose Luis Duran, delphij
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41751

(cherry picked from commit b7528b16c6039a9f0af28f8f3ccdd6ea4eb7e604)
(cherry picked from commit 5623352ec00532193e9cd44332814f8d482422e4)

Approved by: re (gjb)