layers/tcpip: remove dead code in csum computation
layers/tcpip: remove dead code in csum computation

Since the loop exits when csum fits in 16 bits, csum >> 16 is always 0.
It was probably an attempt to remove the loop (9688856 "Add serialization
to a number of layers.")

rouzier/gopacket,rouzier/gopacket,google/gopacket,levigross/gopacket,xuzhaokui/gopacket,xuzhaokui/gopacket,xuzhaokui/gopacket,google/gopacket,google/gopacket,rouzier/gopacket,levigross/gopacket,levigross/gopacket