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
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41751

release/tools/gce.conf

@@ -83,8 +83,6 @@ EOF
 	cat << EOF >> ${DESTDIR}/etc/sysctl.conf
 net.inet.icmp.drop_redirect=1
 net.inet.ip.redirect=0
-net.inet.tcp.blackhole=2
-net.inet.udp.blackhole=1
 kern.ipc.soacceptqueue=1024
 debug.trace_on_panic=1
 debug.debugger_on_panic=0