Remove sig_network_serial

Remove sig_network_serial

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

sig_network_serial is now enpty 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

Change-Id: I525cf894699a1d56f0b3cbb8e630d40ea3549b51
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>