Remove sig_network_serial

Remove sig_network_serial

PerformanceDNS is now optional [1] and "DNS configMap nameserver" makes
operations on coredns which could break a living deployment [2].

sig_network_serial is now empty and then can be safely removed.

[1] https://github.com/cntt-n/CNTT/pull/2580
[2] https://github.com/kubernetes/kubernetes/blob/c262d09bb7d22d62665aa215b5c6d1dd2a865007/test/e2e/network/dns_common.go#L238

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: Ic6d1e700161838c7bed421bf28527b04dc7d588b
Reviewed-on: https://gerrit.opnfv.org/gerrit/c/kuberef/+/72869
Tested-by: jenkins-ci <jenkins-opnfv-ci@opnfv.org>
Reviewed-by: Rihab Banday <rihab.banday@ericsson.com>