icmp: No need to change ip_packets

This has been introduced sometime with 0ddded7070a08fb0bcd00b4f499ddf68bff63a29
(New packetdrill features of this first commit:).

But, it makes actually icmp-tests crash. So, let's get rid of this.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>